Anti-Reflective and Anti-Soiling Coatings with Self-Cleaning Properties

ABSTRACT

The embodiments of the invention are directed to coatings and their uses. More particularly, the embodiments of the invention are directed to coating compositions that include silane-based precursors that are used to form coatings through a sol-gel process. The coatings so formed are characterized by anti-reflective, abrasion resistant, and anti-soiling properties. The coatings also have extended weatherability to heat, humidity, and protection against ambient corrosives. The coatings formed from the compositions described herein have wide application, including, for example, use as coatings on the outer glass of solar cells.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.12/769,580, filed Apr. 28, 2010, which claims the benefit of U.S.Provisional Application No. 61/174,430 filed Apr. 30, 2009. The entiretyof each of the foregoing applications is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The embodiments of the invention are directed to coatings and theiruses. More particularly, the embodiments of the invention are directedto coating compositions that include silane-based precursors that areused to form coatings through a sol-gel process. The resulting coatingsare characterized by anti-reflective, abrasion resistant, andanti-soiling properties. The coatings also have extended weatherabilityto heat and humidity and protection against ambient corrosives. Thecoatings formed from the compositions described herein have wideapplication, including, for example, use as coatings on the outer glassof solar cells or panels.

2. Description of Related Art

Anti-reflective coatings are used in a wide variety of commercialapplications ranging from sunglasses, windows, car windshields, cameralenses, solar panels, and architectural systems. These coatings minimizethe reflections on the surface of the glass as the light rays travelthrough a discontinuous dielectric gradient. The reflection of lightusually results in reduced transmittance of the light across thetransparent material. For optical applications, it is important that amajority of incident light passes through the interface for maximumefficiency. In this context, anti-reflective coatings provide a usefulbenefit in optical applications.

Anti-reflective coatings are normally used in glasses, acrylics, andother transparent materials that serve as windows and glass panelsassociated with architectural structures or energy generating and savingsystems. In building windows, they are used to maximize influx ofincident light to maintain proper lighting or natural ambience as wellas to minimize distracting reflections from glass surfaces. In energygenerating and saving devices, such as solar panels and lightcollectors, the utility of anti-reflective coatings lies in the enhancedefficiency of these devices due to a greater degree of lighttransmittance and, therefore, increased energy generation for the samecost.

In order for optical elements to perform optimally, it is necessary thatthey be free from surface contamination and depositions (e.g., dirt)that may reduce light transmittance and, therefore, performance of thecoatings. In particular, for optical elements that are exposed to anoutside environment, such as solar panels and building windows, the longterm exposure to chemical and physical elements in the environmentusually results in deposition of dirt on the surface of the opticalelement. The dirt may comprise particles of sand, soil, soot, clay,geological mineral particulates, air-borne aerosols, and organicparticles such as pollen, cellular debris, biological and plant-basedparticulate waste matter, and particulate condensates present in theair. Over time, deposition of such dirt significantly reduces theoptical transparency of the optical element. As a result, there isconsiderable expenditure of human and financial resources associatedwith regular cleaning of such optical elements, such as transparentwindows and solar panels.

The deposition of dirt on such optical elements can be classified intotwo types: physically bound and chemically bound particulate matter. Thephysically bound particles are loosely held due to weak physicalinteractions such as physical entanglement, crevice entrapment, andentrapment of particulates within the nanoscale or microscale edges,steps, terraces, balconies, and boundaries on the uneven surface of theoptical element, such as a window surface. These particles can bedislodged with moderate energy forces such as wind, air from amechanical blower, or by means of water flow induced by rain or otherartificially generated sources of flowing water such as a water hose orsprayer. On the other hand, chemically bound particles are characterizedby the presence of chemical interactions between the particlesthemselves and between the particulate matter and the optical elementitself, such as glass or acrylics (e.g., plexiglass) used, for example,in windows. In these cases, removal of these particles becomes difficultand usually requires the use of physical means such as high pressurewater hoses or manual scrubbing or both. Alternatively, chemical meanssuch as the application of harsh solvents, surfactants, or detergents tothe optical element to free the dirt particles from the surfaces can beused.

As noted, the dirt on ambiently exposed optical elements, such aswindows and solar panels, may be somewhat removed based upon naturalcleaning phenomenon such as rain. However, rain water is only effectiveat removing loosely (physically) held particulate matter and is not ableto remove the particulate matter that may be strongly (chemically)bonded to optical element, such as the glass or window surfaces.Furthermore, rain water usually contains dissolved matter that isabsorbed from the environment during its descent that can leave avisible film when dried.

As such, all externally exposed optical elements, such as windowmaterials and solar panels in which the optimal transmission of light isimportant, require some form of routine cleaning efforts associated withtheir maintenance regimen. In fact, the surfaces of these items arecleaned during fabrication as well as routinely during use. The surfacesof these items, such as solar panels, are usually cleaned with water,detergent, or other industrial cleaners. As a result, anti-reflectivecoating materials applied to these optical elements need to be able towithstand the use of normal cleaning agents including detergents, acid,bases, solvents, surfactants, and other abrasives to maintain theiranti-reflective effect. Abrasion of these coatings over time due tocleaning and the deposition of dirt or other environmental particulatemay reduce their performance. Therefore, abrasion resistance is animportant consideration for anti-reflective coatings. For example,resistance to abrasion is an important consideration for a coating usedin connection with a solar panel, particularly for long term functionalperformance of the solar panel.

A majority of anti-reflective coatings are based on oxides as preferredmaterials. Some anti-reflective coatings are made of either a veryporous oxide-based coating or, alternatively, are comprised of stacks ofdifferent oxides. These oxide materials are chemically reactive withdirt particles by means of hydrogen bonding, electrostatic, and/orcovalent interactions depending upon the type of coating material andthe dirt particle. Therefore, these oxide based coatings have a naturalaffinity to bind molecules on their surfaces. Further, highly porouscoatings can physically trap dirt nanoparticles in their porousstructure. As a result, current anti-reflective coatings arecharacterized by an intrinsic affinity for physical and/or chemicalinteractions with dirt nanoparticles and other chemicals in theenvironment and suffer from severe disadvantages in maintaining a cleansurface during their functional lifetime.

Further, one of most common issue frequently associated withanti-reflective coatings is their performance over the entire solarspectrum, particularly with respect to solar panels. While there areseveral anti-reflective coatings that are only effective in a narrowregion of the solar spectrum, for maximum efficiency it is desirablethat anti-reflective coatings perform equally well over the entire solarregion from 300 nm to 1100 nm.

Consequently, there exists a need in the art for a coating that canprovide the combined benefits of anti-reflective properties, such as acoating that can reduce light reflection and scattering from theapplicable optical surface; anti-soiling or self-cleaning properties,such as a coating surface that is resistant to binding and adsorption ofdirt particles (e.g., resistant to chemical and physical bonding of dirtparticles); abrasion resistant properties, such as stability againstnormal cleaning agents such as detergents, solvents, surfactants, andother chemical and physical abrasives; and UV stability or suitableperformance over the entire solar region.

Further, it would be beneficial for such coatings to be mechanicallyrobust by exhibiting strength, abrasion resistance, and hardnesssufficient to withstand the impact of physical objects in theenvironment such as sand, pebbles, leaves, branches, and other naturallyoccurring objects. It would be beneficial for such coatings to alsoexhibit mechanical stability such that newly manufactured coatings orfilms would be less likely to develop cracks and scratches that limittheir optimum performance, thereby allowing such coatings to be moreeffective for a relatively longer term of usage. In addition, it wouldbe beneficial for such coatings to be able to withstand otherenvironmental factors or conditions such as heat and humidity and to bechemically non-interactive or inert with respect to gases and othermolecules present in the environment, and non-reactive to light, water,acid, bases, and salts. In other words, it is desirable to providecoatings having a chemical structure that reduces the interaction of thecoating with exogenous particles (e.g., dirt) to improve the long termperformance of the coating.

It would also be preferable to enable deposition of such coatings ontothe optical surface, such as the surface of a window or solar panelsurface, using common techniques such as spin-coating; dip-coating;flow-coating spray-coating; aerosol deposition; ultrasound, heat, orelectrical deposition means; micro-deposition techniques such asink-jet, spay-jet, or xerography; or commercial printing techniques suchas silk printing, dot matrix printing, etc.

It would also be preferable to enable drying and curing of such coatingsat relatively low temperatures, such as below 150° C. so that thecoatings could be applied and dried and cured on substrates to whichother temperature sensitive materials had been previously attached, forexample a fully assembled solar panel

BRIEF SUMMARY OF THE INVENTION

The present invention provides coating compositions comprising a silaneprecursor or combination of silane precursors, a solvent, optionally anacid or base catalyst, and optionally another additive. The coatingcompositions are hydrolyzed to provide a sol that can be coated on asubstrate from which a gel is formed that is subsequently dried andcured to form a coating having a combination of anti-reflectiveproperties, anti-soiling or self-cleaning properties, and abrasionresistance. Accordingly, the anti-reflective coatings provided by thepresent invention are physically, mechanically, structurally, andchemically stable.

In some embodiments, the coating compositions include a combination ofsols containing tetraalkoxysilane, organosilane, and organofluorosilanethat can be used to form coatings for transparent substrates such assolar panels. In some embodiments, the composition of the coatingcomposition is based upon a precise selection of solvent, pH, solvent towater ratio, and solvent to silane ratio that allows the resulting solto remain stable for a significant period of time without exhibitingchange in its chemical or physical characteristics.

The invention also provides methods for applying the coatings of thepresent invention and for using such coatings. In some embodiments, themethods of treating a substrate comprises pre-treatment of the substratebased on combination of chemical treatment, etching, and/or polishing orcleaning steps that enable a uniform spreading of the sol for making athin film or coating with thickness ranging from 50 nm to 200 nm.Thereafter, in some embodiments, the methods include applying the sol tothe substrate and allowing the sol to gel to form the coating with thedesired properties. In some embodiments, the application of the sol tothe substrate includes drop rolling and/or flow coating that results inuniform deposition of the sol to form an even, uniform and crack-freecoating. In some embodiments, the method includes thermally treating thecoated articles under specific condition of heat and humidity to form achemically durable coating that adheres strongly to the substratewithout cracking and/or peeling.

In some embodiments, the invention provides for the use of the coatingcompositions as an efficiency enhancement aid in architectural windowsin building and houses by the provision of anti-reflection benefitsand/or by the provision of anti-soiling benefits to augment theanti-reflection benefits. In other embodiments, the invention providesfor the use of the coating compositions as an efficiency enhancement aidin treatment of transparent surfaces (that require regular cleaning) tomake them self-cleaning.

In some embodiments, the invention provides a coated glass-based articlesuitable for use as outer cover of a solar module assembly that isanti-reflective, hydrophobic and/or oleophobic, and exhibits resistanceto abrasion, UV light, heat, humidity, corrosives such as acids, bases,salts, and cleaning agents such as detergents, surfactants, solvents andother abrasives.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 2% with coatings on glass slides made fromthe composition given in Example 1;

FIG. 2 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 3.4% with coatings on a glass slidessubstrate made from the composition given in Example 2;

FIG. 3 a is an SEM cross-sectional view of a coating made from thecomposition of Example 1 on a glass slide substrate;

FIG. 3 b is a SEM oblique view of a coating made from the composition ofExample 1 on a glass substrate;

FIG. 4 a is an SEM cross-sectional view of a coating made from thecomposition of Example 2 on a glass slide substrate;

FIG. 4 b is a SEM oblique view of a coating made from the composition ofExample 2 on a glass substrate;

FIG. 5 shows an XPS spectrum of a coating made from the composition ofExample 1;

FIG. 6 shows an XPS spectrum of a coating made from the composition ofExample 2;

FIG. 7 a illustrates nano-indentation data showing the indenter depthprofile and hardness of a coating made from the composition of Example 1on a glass slide substrate;

FIG. 7 b illustrates nano-indentation data showing the indenter depthprofile and Young's Modulus of a coating made from the composition ofExample 1 on a glass slide substrate;

FIG. 8 a illustrates nano-indentation data showing the indenter depthprofile and hardness of a coating made from the composition of Example 2on a glass slide substrate;

FIG. 8 b illustrates nano-indentation data showing the indenter depthprofile and Young's Modulus of a coating made from the composition ofExample 2 on a glass slide substrate;

FIG. 9 illustrates the results of accelerated soiling studies on lighttransmission for coatings made according to some embodiments of thepresent invention; and

FIG. 10 illustrates the results of accelerated soiling studies on dirtadhesion for coatings made according to some embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention are described below in conjunctionwith the Figures; however, this description should not be viewed aslimiting the scope of the present invention. Rather, it should beconsidered as exemplary of various embodiments that fall within thescope of the present invention as defined by the claims. Further, itshould also be appreciated that references to “the invention” or “thepresent invention” should not be construed as meaning that thedescription is directed to only one embodiment or that every embodimentmust contain a given feature described in connection with a particularembodiment or described in connection with the use of such phrases. Infact, various embodiments with common and differing features aredescribed herein.

The present invention is generally directed to coatings that provide acombination of benefits including anti-reflective properties,anti-soiling properties, self cleaning properties and manufacturingflexibility as well as other benefits. Accordingly, the coatings of thepresent invention may be used on substrates, such as transparentsubstrates, to increase the light transmittance through the substrates.In particular, the coatings may be used on transparent substrates suchas glass or the front cover glass of solar panels.

The present invention is particularly well suited for use with glassused in solar energy generation (“solar glass”). It should be understoodthat solar energy generation includes solar photovoltaic and solarthermal, wherein solar insulation is used to produce heat either as anend-point or as an intermediate step to generate electricity.Furthermore it should be understood that solar glass may be used in anyapplication where maximal transmission of solar energy through the glassis desired such as for example in greenhouses. Typically solar glass ishigh transmission low iron glass. It may be either float glass, that is,flat glass sheets formed on a molten tin bath, or rolled glass whereinthe flat glass is formed by the action of rollers. Float glass is oftencharacterized by the presence of tin contamination on the bottom (“tinside”) of the glass. Rolled glass is typically textured on one side toimprove its performance in solar panels. It may also be formed intotubes such as those used as receivers in solar thermal energy generationor in some forms of solar photovoltaic generation such as, for example,that offered by Solyndra. The present invention may also be applied toglass surfaces used as mirrors in solar energy generation such asparabolic trough systems or in heliostats. It may also be used to coatvarious glass lenses such as Fresnel lenses used in solar thermalgeneration. Additionally, solar glass may have various coatings applied.For example a common coating is a transparent conduction oxide such asIndium Tin Oxide (ITO) on one side of the glass. This coating is used toprovide the front electrode for many thin film solar panel technologies.Other coatings may be present such as coatings to seal in alkali ionssuch as Na⁺ and Ca⁺ that are used in the manufacturer of the glass butthat cause long term reliability problems when leached out by water.Other techniques to solve this problem are to deplete these ions in thinlayers of the glass surface. Solar glass may also be coated with areflective surface to form a mirror. Solar glass may be tempered oruntempered. Tempered glass is significantly stronger, and solar panelsmanufactured using it typically only need one sheet of glass. Solarpanels manufactured with untempered front glass typically need a backsheet of tempered glass to meet strength and safety requirements. Manythin-film solar photovoltaic technologies also use the front glass as asubstrate upon which they deposit materials that comprise the solarcell. The processes used during the manufacturer of the solar cell mayadversely affect the properties of any existing coatings on the glass,or existing coatings may interfere with the solar cell manufacturingprocess. The present invention is completely tolerant of the type ofglass selected by the solar panel manufacturer. It works equally well onfloat or rolled glass. It is not affected by the presence tincontamination on float glass.

One critical issue for solar panel manufacturers that use ITO (orsimilar) coated glass is tempering. It is very difficult to achievelow-cost, high quality ITO coated tempered glass. Therefore solar panelmanufacturers that requite ITO coated glass use untempered glass,necessitating the use of a second sheet of tempered glass on the backside of the solar panel. Additionally, even if suitable ITO coatedtempered glass was available, some thin-film solar manufacturingprocesses heat the glass during manufacture to the extent that thetemper is lost. All existing anti-reflective glass on the market istempered, because the anti-reflective coating is actually formed duringthe tempering process. Tempering is the process by which the glass isheated to 600° C. to 700° C., and then quickly cooled. This hightempering temperature effectively sinters the anti-reflective coatingproviding it with its final mechanical strength. Thus, solar panelmanufacturers that cannot use tempered glass, typically cannot useanti-reflective glass. The present invention may be applied and cured ata low temperature of between 20° C. and 200° C. and between 20° C. and130° C. and further between 80° C. and 200° C. This low temperaturefacilitates the coating of completed solar panels without damage to thepanel. Thus it is an anti-reflective solution for users of untemperedsolar glass.

The low temperature curing of the coatings of the present invention alsoprovides substantial benefits to solar panel manufacturers beyondenabling untempered anti-reflective glass. By making possible thecoating of the glass without the need for the tempering step, solarpanel manufacturers are enabled to apply their own anti-reflectivecoating. Currently, the requirement for a large tempering oven meansthat solar panels manufacturers are restricted to buying anti-reflectiveglass from glass manufacturers. This means that they must maintaininventory of both anti-reflective coated and non-coated glass. As thesecannot be used interchangeably, inventory flexibility is reducednecessitating keeping larger amounts of inventory on hand. The abilityfor the solar panel manufacturer to apply their own coating means thatthey can just hold a smaller inventory of non-coated glass and thenapply the anti-reflective coating to that as needed.

In addition, existing anti-reflective coatings are prone to scratchingduring the solar panel manufacturing process. Typically solar panelmanufacturers must use a plastic or paper sheer to protect the coating.As the coating of the present invention can be applied to fullymanufactured solar panels, it can be applied at the end of themanufacturing process thus removing the need for the protection sheetand the opportunity for damage to the coating during manufacture.

Existing anti-reflective coatings from different manufacturers tend tohave subtle color, texture and optical differences. This presentsproblems to solar panel manufacturers who desire their products to havea completely consistent cosmetic finish. If they manufacturer largenumbers of solar panels it is almost inevitable that they will have toorder anti-reflective glass from different suppliers causing slightdifferences in the appearance of the final products. However, thecoating of the present invention enables solar panel manufacturers toapply their own coating and so enables cosmetic consistency over anunlimited number of solar panels.

In addition, to their anti-reflective properties, the coatings describedherein exhibit anti-soiling and/or self-cleaning properties, as they areresistant to the adhesion of dirt and promote the removal of any adhereddirt by the action of water. More specifically, the coatings describedherein are characterized by extremely fine porosity that minimizes thedeposition of dirt by physical means. Further, these coatings arecharacterized by a low energy surface that resists chemical and physicalinteractions and makes it easy to dislodge the particles, thereby makingthe surfaces essentially anti-soiling. The reduced physical and/orchemical interactions with the environment, such as dirt, make theexposed surface of these coatings less susceptible to binding of dirtand also make it easier to clean with a minimal expenditure of force orenergy.

Typically, in order to completely clean ordinary glass, a mechanicalaction, for example brushes or high pressure jet, is required todislodge dirt that is strongly adhered to the surface. However, thecoatings of the present invention present a surface such that dirt ismuch more attracted to water then to the surface. Thus in the presenceof water any dirt resting on the surface is efficiently removed withoutthe need for mechanical action. This means that coated glass willachieve a high level of cleanliness in the presence of natural orartificial rain without human or mechanical intervention. In addition,the amount of water required to clean the glass is substantiallyreduced. This is of special significance given that the most effectivelocations for solar energy generation tend to be sunny warm and arid.Thus, water is a particularly expensive and scarce resource in the verylocations that solar energy is most effective.

The present invention enables a significant reduction in the LevelizedCost of Energy (LCOE) to the operator of a solar energy generatingsystem. First, the anti-reflective property increases the efficiency ofthe solar panels. Increased efficiency enables a reduction of cost inthe Balance Of System (BOS) costs in construction of the solar energygeneration system. Thus, for a given size of system the capital costsand construction labor costs are lower. Second, the anti-soilingproperty increases the energy output of the solar panels by reducing thelosses due to soiling. Third the Operating and Maintenance (O&M) costsare reduced because fewer or no washings are needed eliminating laborand water cost associated with washing.

The coatings described herein also contain water and oil resistanthydro/fluoro-carbon groups that make them chemically non-reactive andnon-interacting. When used in combination with a glass substrate, thecoatings bind to the glass surface using siloxane linkages that makethem adhere strongly and makes them strong, durable, and abrasion andscratch resistant. In summary, these coatings are physically andchemically nonreactive, mechanically and structurally stable,hydrophobic, oleophobic, and stable across the UV spectrum. Accordingly,it should be appreciated that the coatings described herein haveparticular application to transparent substrates that are exposed to theenvironment, such as exterior windows and the front cover glass of solarpanels.

Generally, the coatings described herein are prepared by a sol-gelprocess. The starting composition, referred to as a “coating mixture” or“coating composition,” includes a silane precursor or a combination ofsilane precursors that when hydrolyzed and condensed forms a particulatesuspension of particles in a liquid sol. This sol can be coated onto asubstrate using coating techniques known in the art, gelled to form agel, and dried to form a hard layer or coating having the propertiesnoted above. The process of curing the dried gel further hardens it.

Generally, the resulting properties of the coating described above areprovided by using a particular combination of components in theformation of the final coating. In particular, the selection of aparticular silane precursor or mixture of silane precursors incombination with other components in the coating mixture is important inproviding a coating with the desired properties. For example, in someembodiments, the coatings are made from a mixture of silane precursorsincluding alkoxysilane, organosilane, and organofluorosilane. In someembodiments, separate coating mixtures or mixtures of silane precursorscan be used to form separate sols that may then be combined to form afinal sol that is applied to a substrate to be coated. Further, a singlesol, or separately prepared sols that are combined together, may becombined with another silane precursor to form a final sol that isapplied to a substrate to be coated.

It should be appreciated that the coating mixture may include othercomponents in addition to any silane precursors. For example, the silaneprecursors may be mixed with water or a solvent mixed with water, anacid or base catalyst, and one or more composition modifying additivesto impart, provide, modulate, and/or regulate the intrinsic structures,properties, function, and performance of the resulting coatings. Thecomposition modifying additives may include particle size modifyingadditives, cross-linking additives, and surface modifying additives.Each of these components in the coating mixture is described in moredetail below.

It should be appreciated that long term sol stability, as defined by theretention of a liquid state without gelation or precipitation, isimportant in practical applications. Accordingly, the sols provided bythe present invention are chemically and physically stable under ambientstorage conditions for periods ranging from about 3 months to about 6months. In some embodiments, the sol is stable for periods ranging fromabout 6 months to about 9 months when stored at 4° C. The stability ofthe sol is due to several factors including the specific combination ofthe silane precursors, control of the pH of the sol, selection of asolvent with a balance of hydrogen bonding, polar, and nonpolar groups,balance of the solvent to water ratio in the coating mixture, andbalance of the silane(s) to solvent ratio, each of which is described inmore detail below.

Each of the specific components used in the coating mixture will now bedescribed. As noted, a silane precursor or a combination of silaneprecursors is used in the coating mixture to generate the coatings ofthe present invention. The silane precursor used to make the coating maybe selected based upon the properties desired to be imparted to theresulting coating. In some embodiments, the silane precursor is selectedbased upon its tendency to adhere to glass due to formation of strongSi—O—Si bonds between the surface of the glass and the components of thecoating composition.

For example, tetraalkoxysilanes when hydrolyzed form an extensivelycross-linked structure due to the formation of four Si—O—Si linkagesaround each silicon atom. These structures are characterized bymechanical stability and abrasion resistance. To impart hydrophobicityto the ultimate coating, organically-modified silanes (such asmethyltrimethoxysilane) can be used in addition to thetetraalkoxysilane. Further, to impart oleophobicity and anti-soilingcharacteristics, organofluorosilanes can be used in addition to thetetralkoxysilane.

In some embodiments, the silane precursor can be alkyltrialkoxysilane,tetraalkoxysilane, an organosilane, an organofluorosilane, or acombination of any one or more of these. The alkyl group foralkyltrialkoxysilane can be methyl, ethyl, or propyl. Thetetraalkoxysilane is of the type (OR)₄Si where R=H, methyl, ethyl,isopropyl, or t-butyl. The tetraalkoxysilane can be a methoxy, ethoxy,or isopropoxy or t-butoxy analog. The organosilane is of the type(OR)₃Si—R′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andR′=methyl, ethyl, or propyl. The organosilane precursormethyltrimethoxysilane can be substituted with triethoxy ortri-isopropoxy derivatives. The organofluorosilane is of the type(OR)₃Si—Rf′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andRf′=3,3,3-trifluoropropyl, or tridecafluoro-1,1,2,2-tetrahydrooctyl. Theorganofluorosilane precursor can be(3,3,3-trifluoropropyl)trimethoxysilane or(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane.

In other embodiments, the coating mixture may comprise organosilicamonomers, oligomers, particles, polymers, and/or gels of organosilicatematerials made from a single component or mixtures of starting materialshaving the general formula (X)_(n)Si(R)_(4-n) and/or(X)_(n)Si—(R)_(4-n)—Si(X)_(n) where X comprises halides, alkoxides,carboxylates, phosphates, sulfates, hydroxides, and/or oxides; n=1, 2,or 3; and R is an alkyl, alkenyl, alkynyl, phenyl, or benzenylhydrocarbon and/or fluorocarbon chain with 1-20 carbon atoms and asolvent to dissolve or disperse the organosilane.

As noted, in some embodiments, combinations of silane precursors can beused in the coating mixture. The ratio of these silane precursors can bevaried independently to fine tune and modify the overall characteristicsof the ultimate coating. As such, the stoichiometric ratio of the silaneprecursors is important for the stability of the sol made from theseprecursors, as well as for the function and performance of films andcoatings made from these sols. For example, the relative ratio of eachof these precursors is important to form a stable cross-linked networkfor mechanical stability and abrasion resistance.

In some embodiments three silane precursors are used. In someembodiments, the three silane precursors used may include analkoxysilane, an organosilane, and an organofluorosilane. In someembodiments, the alkoxysilane may be alkyltrialkoxysilane ortetraalkoxysilane. The alkyl group for alkyltrialkoxysilane can bemethyl, ethyl, or propyl. The tetraalkoxysilane is of the type (OR)₄Siwhere R=H, methyl, ethyl, isopropyl, or t-butyl. The tetraalkoxysilanecan be methoxy, ethoxy, or isopropoxy or t-butoxy analogs. Theorganosilane may be of the type (OR)₃Si—R′ where R=H, methyl, ethyl,isopropyl, or t-butyl, and R′=methyl, ethyl, or propyl. The organosilaneprecursor methyltrimethoxysilane can be substituted with triethoxy ortri-isopropoxy derivatives. The organofluorosilane may be of the type(OR)₃Si—Rf′ where R=H, methyl, ethyl, isopropyl, or t-butyl, andRf′=3,3,3-trifluoropropyl, or tridecafluoro-1,1,2,2-tetrahydrooctyl. Theorganofluorosilane precursor can be(3,3,3-trifluoropropyl)trimethoxysilane or(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane. In someembodiments, the three silane precursors used may includetetramethoxysilane or tetraethoxysilane (as the tetraalkoxysilane),methyltrimethoxysilane (as the organosilane), andtrifluoropropyltrimethoxysilane ortridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane (as theorganofluorosilane). In some embodiments, the total concentration of allof the silane precursors may be from about 1% by weight to about 10% byweight. In some embodiments, the total concentration of all of thesilane precursors may be from about 1% by weight to about 4% by weight.

Aside from the total concentration of all of the silane precursors used,the relative amount of each is also important. For example, in theembodiment with three silane precursors, the relative amounts of eachthat are added to the coating mixture is based upon several factors,including visual sol homogeneity in the coating mixture, uniformity ofthe film or coating once deposited on the substrate, and the desiredabrasion resistance of the coating. More specifically, the relativeamounts of each of the three silanes are adjusted to fine tune solhomogeneity, adjust film or coating uniformity, and to control abrasionresistance of the final coating. It should be appreciated that therelative amounts of the silanes used are determined regardless ofwhether the silane precursors are added to the same coating mixture orwhether separate coating mixtures are used to generate separate solsthat are then combined to form a final sol that is applied to a givensubstrate. In other words, the relative amounts of the silane precursorsis determined based upon the amount of each silane precursor present inthe final sol that will be applied to a given substrate.

Regarding sol homogeneity, if three silane precursors are used, eachwould be hydrolyzed in a solvent medium to form a sol. However, analkoxysilane such as tetraalkoxysilane is polar and hydrophilic whilethe organosilane and the organofluorosilane are hydrophobic. As aresult, they exhibit differential interaction with each other and withthe solvent matrix. High amounts of the organofunctional silanes(wherein “organofunctional” refers to any trialkoxysilane with an Rgroup, which includes both organo and organofluorosilanes) lead to phaseseparation and aggregation while lower amounts of organofunctionalsilanes leads to films with poor anti-reflective and anti-soilingcharacteristics. Therefore, using the appropriate relative amounts ofeach of the silane precursors is essential for the desired function andapplication of the coating mixture on a substrate made from the sol.

Regarding film or coating uniformity, the final sol is applied to asubstrate followed by gelation and evaporation of the solvent to form asilica-based coating. Due to their differential interactions with thesolvent matrix, the particles made from the different silane precursorsdiffer in their solubility. High amounts of the organofunctionalizedsilanes (organosilane plus organofluorosilane) lead to development ofpatchy opaque films due to their limited solubility and phase separationduring solvent evaporation. On the other hand, high amounts oftetralkoxysilanes lead to formation of films that do not exhibitsufficient anti-reflective or anti-soiling characteristics.

Regarding abrasion resistance, the organosilanes and organofluorosilaneseach are capable of forming three Si—O—Si linkages, while thetetraalkoxysilanes can form four Si—O—Si linkages. Therefore, therelative ratio of each of these precursors is important in forming astable cross-linked network for mechanical stability and abrasionresistance. High amounts of the organofunctionalized silanes(organosilane plus organofluorosilane) in the films result in films thatare not sufficiently mechanically stable and that lack sufficientabrasion resistance.

In some embodiments, the relative weight percent ratio of thealkoxysilane, such as tetraalkoxysilane, to the total amount offunctionalized silanes (organosilane plus organofluorosilane) lies inthe range of about 0.2 to about 2. In some embodiments, this ratio is inthe range of about 0.2 to about 1, and in some embodiments, it is in therange of about 0.5 to about 1.

Similarly, the relative ratio of organosilane to organofluorosilane isimportant in obtaining a stable sol that can spread evenly on thesubstrate and that does not result in the precipitation or aggregationthat may reduce optical transmission through the coating, for example,if used in connection with a solar panel cover. The relative amounts oforganosilane and organofluorosilane also determine the hydrophobicityand anti-soiling characteristics. The structural similarities of thesetwo precursors as well as their relative solvation and theirintermolecular interactions control their interactions with each otherrelative to any solvent used in the coating mixture as described below.For example, the chain lengths of the organosilane and fluorosilane areimportant in determining the ratio of these components. In general, ifthe side chains on these precursors get too big, then a high amount ofthese precursors would cause opaque films. Therefore, for precursorswith a large R group on the organosilane, its amount would need to bereduced relative to fluorosilane and alkoxysilane. The same holds truefor fluorosilane as well.

In some embodiments, the relative weight percent ratio of organosilaneto organofluorosilane is in the range of about 0.5 to about 1.5. In someembodiments, this ratio is in the range of about 0.75 to about 1.25, andin some embodiments it is in the range of about 0.5 to about 1.Depending upon the use of the ultimately formed coating, in someembodiments, the amount of organosilane or mixture of organosilanes canvary typically from about 0.1% to about 90%, from about 10% to about65%, and from about 10% to about 25%, by weight of the coatingcomposition or mixture. Examples of organofluorosilanes that can be usedinclude tridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane;tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane;3,3,3-trifluoropropyl)trimethoxysilane;3,3,3-trifluoropropyl)trichlorosilane; and3,3,3-trifluoropropyl)methyldimethoxysilane. Examples of organosilanesthat can be used include methyltrichlorosilane, methyltrimethoxysilane,phenylmethyldiethoxysilane, dimethyldimethoxysilane, andn-propylmethyldichlorosilane. It should be appreciated that theforegoing fluorosilanes and organosilanes may be used alone or in anycombination.

As noted, in some embodiments, a solvent may be used in the coatingmixture with the silane precursor or combination of silane precursorsdescribed above. For example, a solvent may be used in a coating mixturethat is used to form a single sol, or a solvent may be used inconnection with the formation of separate sols that may be combined toform a final sol for coating a given substrate.

Generally, the solvent is an organic solvent that is in a mixture withwater. The nature and amount of the solvent along with the solvent/waterratio and the solvent/silane precursor ratio used in the coating mixtureis important in the stability of the sol, as well as in imparting thespecific function and performance of the coatings made from the solcontaining the hydrolyzed silane precursors. The choice of solvent alsoaffects the viscosity of the sol or spreadability of the sol on thesubstrate to be coated, as well as the intermolecular interactions ofthe sol particles and their interactions with the substrate surface tobe coated. In addition to providing a matrix for carrying out thesol-gel reaction, the solvent also influences the reaction kinetics ofthe sol formation steps. Each of these effects on the selection of asolvent and the amount of solvent to be used is described in more detailbelow.

One factor is the effect of the solvent on hydrolysis and condensationof the silane precursors. In the sol-gel process the silanes arehydrolyzed to silanols that further condense to form siloxane linkages.The particle sizes and the consequent viscosity of the sol depends uponthe number of particles and the state of their aggregation in the sol. Asolvent that exhibits strong hydrogen bonding and has hydrophilicproperties can stabilize individual sol particles to make the sot lessviscous and more stable for a longer period of time. A less viscous solcan form thin films on the order of 50-200 nm that are beneficial inproviding a coating that provides an anti-reflective effect.

Another factor is the effect of the boiling point and rate ofevaporation of the solvent on formation of a uniform film on thesubstrate from the sol. Solvents with relatively low boiling points and,therefore, having an appreciable rate of evaporation are more desirablein forming a uniform film from the coating mixture or sol. As thesolvent evaporates, the silicate particles come closer together and forma network. If the solvent evaporates too rapidly the films may not beuniform. Similarly, a solvent with high boiling point does not evaporateat an appreciable rate under ambient conditions, thereby leading toformation of less desirable opaque films with larger particles.

The solvent also affects the overall spreadability of the sol during theapplication of the sol on the substrate since the solvent affects thesurface energy of the sol. Solvents with lower surface tension enablemore even spreading of the sol onto the substrate. Accordingly, in someembodiments, the solvent provides a balance of both hydrophobic andhydrophilic groups to stabilize the sol and to provide for an optimumsol viscosity. For example, alcohols with a moderate chain length of 3-6carbon atoms are particularly advantageous in lowering the surfacetension in the sol.

Another factor is the effect of hydrogen bonding and otherintermolecular interactions on the stability of the sol. Solvents thatcan participate in hydrogen bonding interactions stabilize the solparticles by preventing aggregation and precipitation when the sol isdeposited onto a substrate. Solvents containing hydrogen bond donor —OHfunctional groups, such as alcohols and glycols, along with solventscontaining hydrogen bond acceptor R—O—R linkages, such as ethers, areparticularly advantageous in preparation of stable sols.

Another factor is the effect of a solvent's toxicity, volatility, andflammability. The toxicity and flammability of the solvent can be aconcern in the long term storage and transportation of the sol.Therefore, solvents with low toxicity, volatility, and flammability arepreferred. Alcohols, glycols, and glycol ethers are particularlyadvantageous in this regard.

Considering the above factors in the selection of a solvent, in someembodiments, organic solvents that may be used are ones that containhydrogen bond donor groups, such as alcohols and gylcols with —OHgroups, and hydrogen bond acceptor groups, such esters (—COOK), ethers(R—O—R), aldehydes (RCHO), and ketones (R₂C═O). Another group ofsolvents that may be used include glycol ethers and glycol ether esters.These solvents provide a balance of the factors outlined above forstability, viscosity, and spreadability of the sol. In some embodiments,the solvent that may be used in the coating mixture includes methanol,ethanol, 1-propanol, 2-propanol (isopropanol), n-butanol, isobutanol,tert-butanol, pentanol, hexanol, cyclohexanol, ethylene glycol,propylene glycol, ethylene glycol monomethyl ether, ethylene glycoldimethyl ether, propylene glycol monomethyl ether, diglyme, acetone,methyl ethyl ketone (butanone), pantanones, methyl t-butyl ether (MTBE),ethyl t-butyl ether (ETBE), acetaldehyde, propionaldehyde, ethylacetate, methyl acetate, ethyl lactate, and any combination of any ofthe foregoing. In other embodiments, the organic solvent may be aketone. In other embodiments, the solvent may be acetone, ethanol,propanol, isopropanol, butanol, t-butanol, etc. It should be appreciatedthat the foregoing solvents may be used alone or in any combination.

As noted, the solvent may be used in combination with water. Therelative ratio of the solvent to water is important to obtain thedesired particle sizes of the sol, which, in turn determines theviscosity of the sol and the resulting thickness of the films formedfrom the sol once applied to a substrate. As noted above, the thicknessof the film is important in providing a coating having suitableanti-reflective properties.

In addition, the relative amount of solvent to water and the solcomposition play a key role in stabilizing sol particles so that theparticles remain effectively suspended in the solvent matrix withoutaggregation. Specifically, a balance of hydrophobic and hydrophilicgroups is necessary to provide a sol that is stable for extendeddurations, which in some embodiments may be more than four months, suchthat the sol does not exhibit large changes in its properties and canstill be spread evenly on the substrate, such as a glass surface. Thisstability is also important in enabling the proper spreading of the solduring its application to a substrate.

Essentially, the solvent to water ratio dictates the balance ofhydrophobic and hydrophilic sites in the matrix of the sol and thesolvent. Hydrolysis of silane precursors forms hydroxylated particlescontaining silanol groups. These silanol groups are effectivelysolubilized by a hydrophilic solvent that provides hydrogen bonding andmay reduce cross-linking resulting from condensation reactions.Condensation of these silanol groups to form non-polar Si—O—Si linkagesresults in cross-linking of the particles, and, due to a reduction insurface hydroxyl groups, the cross-linked structures are energeticallyfavored in a medium that is more hydrophobic. Similarly, thecondensation reaction causes the release of water molecule, which isretarded in a medium with high water content. Conversely, a medium withhigh water content favors hydrolysis of silane precursors to formhydrolyzed sol particles that can combine to form a precipitate.Therefore, the solvent to water ratio is important to form a stable sol.

In particular, the solvent to water ratio plays a significant role insolubilization of the sol particles containing, for example, thosegenerated from silane, organosilane, and organofluorosilane precursors.The nature of the silane precursors dictates a range of functionalgroups on the surface of sol particles that range from hydrophilic andhydrogen bond donating Si—OH groups to polar and hydrogen bond acceptingSi—O—Si groups to nonpolar and hydrophobic organo and fluoroalkylgroups. The solubility and interparticle interactions depend stronglyupon the composition of the solvent matrix especially, as noted, thehydrophobic and hydrophilic balance of the solvent matrix.

Accordingly, in some embodiments, the solvent to water weight percentratio in the coating mixture that has been found to be most advantageousto the stability and spreadability of the sol is in the range of about 5to about 20. In some embodiments, the solvent to water weight percentratio is in the range of about 6 to about 12, and in some embodiments itis in the range of about 7 to about 10.

The ratio of the combined weight percent of solvent, including in thiscase the water as well, (i.e., organic solvent(s) plus water) to that ofthe combined weight percent of the silane precursor (i.e., the total ofall silane precursors used in the coating mixture if more than onesilane precursor is used, regardless of whether the silane precursorsare added together to form one sol or whether separate sols are formedthat are then combined) plays a important role in the long termstability and functional effectiveness of the coatings made from thecoating mixtures. Specifically, this ratio can affect several aspects ofthe sol, including the sol particle size, viscosity, and stability.

The solvent is used to solubilize the silane precursors as well as toeffectively disperse the sol particles. A reaction medium that containsa high amount of silane precursor has a high effective concentration ofsilanes that accelerate the hydrolysis reaction to form smallerparticles, while a reaction medium containing a lower concentration ofsilane precursor enables formation of larger particles in the sol. Solscontaining very small particles, such as particles less than 10 nm insize, result in transparent films; however, they do not exhibit anyanti-reflective effects. Sols containing larger particles, such asparticles larger than 100 nm in size, produce films that tend to besemi-transparent or opaque. Therefore, in some embodiments, solparticles having a size in the range of 10-50 nm are desired.

The relative ratio of solvent to silane also affects viscosity andsurface tension of the sol. Highly viscous sols with a smaller solventto silane precursor ratio can form coatings that are too thick, whilevery low viscosity sols with a very high solvent to silane precursorratio results in patchy films and coatings. Therefore, the relativeamount of solvent to silane precursors in the sol is important inmaintaining proper consistency of the sol and to provide a sufficientmass of silane to form films and coatings with desired thicknesses. Insome embodiments, the ratio of solvent to silane precursor is controlledto produce coatings having a thickness of about 60 nm to about 150 nm,which is important to provide an anti-reflective effect.

The relative ratio of solvent to silane precursor is also important incontrolling the isolation and separation of sol particles to preventinterparticle aggregation and agglomeration. At high ratios of solventto silane precursor (i.e. low concentrations of silane precursorrelative to the solvent) the particles are effectively dispersed by thesolvent matrix, which keeps them separate and reduces or preventsaggregations. However, the films made from such sols lack theappropriate thickness to impart useful functional benefits ofanti-reflection, anti-soiling, and abrasion resistance. At low ratios ofsolvent to silane precursor (i.e., high concentration of silanesrelative to solvent), the particles are not effectively solubilized andsuspended in the solvent matrix resulting in aggregation andprecipitation. Coatings made from these sols are opaque or semi-opaqueand generally are not suitable for use as anti-reflective coatings.

Therefore, it should be appreciated that a precise balance of solvent tosilane precursor is important for the stability of the sol, as well asthe function and performance of the coatings made from sols. In someembodiments, this effective weight percent ratio of total solvent (i.e.,water plus organic solvent) to total silane precursors is in the rangeof about 25 to about 125. In some embodiments, this ratio is in therange of about 40 to about 99, and in some embodiments, this ratio is inthe range of about 50 to about 75. The range for this ratio applies toeach coating mixture used to form a sol that may be combined to form thefinal sol that is applied to a given substrate.

In light of the above considerations and ratios, the amount of solvent,including in this case the water as well, (i.e., organic solvent(s) pluswater) in the overall coating mixture from which the sol is formed canvary from about 50% to about 90% by weight. In some embodiments, theamount of solvent in this mixture may be from about 80% to about 90% byweight. In one embodiment, a mixture of silane precursors including atetraalkoxysilane, an organosilane, and an organofluorosilane may beused with isopropanol as the organic solvent in water. In this case, theamount of isopropanol and water may be from about 80% to about 90% byweight. In one embodiment, a mixture of silane precursors includingtetramethoxysilane or tetraethoxysilane, methyltrimethoxysilane, andtrifluoropropyltrimethoxysilane may be used with isopropanol as theorganic solvent in water, the amount of solvent may be 86.4% by weight.The range for this ratio also applies to each coating mixture used toform a sol that may be combined to form the final sot that is applied toa given substrate.

The silane precursor(s) and solvent may also be mixed with an acid or abase as a catalyst to accelerate hydrolysis of the silane precursors.The nature and amount of the acid or base used is important for thestability of the sol as well as function and performance of the coatingsmade from the sol containing hydrolyzed silane precursors.

When acids are used as catalysts, they can be either a weak acid or astrong acid. They can also be organic acids or inorganic acids. Whenbases are used as catalysts, they can be either a weak base or a strongbase. Similarly, they can also be organic bases or inorganic bases.Acids and bases persist in the sol after the reactions. Examples ofstrong acids include hydrochloric acid and nitric acid. Examples of weakacids include acetic acid, trifluoromethanesulphonic acid, and oxalicacid. Examples of strong bases include potassium hydroxide, sodiumhydroxide, and aqueous ammonia. Examples of weak bases include pyridine,tetraethylammonium hydroxide, and benzyltriethylammonium hydroxide.

Since the acid or base catalyst persists in the sol, their concentrationin the medium controls the final pH of the system. For maximum stabilityof the sol, the pH of the final sol should be controlled to preventaggregation and precipitation, since the sol particles otherwise have atendency to aggregate and from viscous gels or precipitates. Similarly,the pH of the sol has been determined to be important in maintaining aproper range of pH for long term sol stability. At low pH the silaneparticles are positively charged and, therefore, can stay separated dueto interparticle electrostatic repulsions. At high pH, there is atendency to form anionic silicates that do not form a network.

Two distinct regions of pH stability have been identified that enhancethe long term stability and durability of the sol. With acid catalysts,in some embodiments, the effective pH is in the range of about 1 toabout 4. In some embodiments, this effective pH is in the range of about2 to about 4, and in some embodiments, this effective pH is in the rangeof about 2.5 to about 3.5. With base catalysts, in some embodiments, theeffective pH is in the range of about 7 to about 10. In someembodiments, this effective pH is in the range of about 7 to about 9,and in some embodiments, this effective pH is in the range of about 7.5to about 8.5.

In some embodiments, depending upon the application of the sol, theamount of the catalyst can vary from 0.001% to about 2%, from about 0.1%to about 1%, and from about 0.01% to about 0.1%, by weight of thecoating mixture from which the sol is made. Nonlimiting examples ofuseful catalysts are HCl, HNO₃, H₂SO₄, H₃PO₄, protonated amines such asN(H/R)_(3-n)[HX] where X comprises halides, nitrates, phosphates, orsulfates; n=0, 1, or 2; and R comprises an alkyl hydrocarbon chain, andcombinations of the foregoing.

In one embodiment, a mixture of silane precursors including atetraalkoxysilane, an organosilane, and an organofluorosilane may beused with isopropanol as a solvent in water and with 0.04 M HCl (acid)or 0.05 M NH₄OH (base) as a catalyst. In one embodiment, a mixture ofsilane precursors including tetramethoxysilane or tetraethoxysilane,methyltrimethoxysilane, and trifluoropropyltrimethoxysilane may be usedwith isopropanol as a solvent in water and with 0.04 M HCl (acid) or0.05 M NH₄OH (base) as a catalyst. In some embodiments that containprehydrolyzed organosilanes as the silane precursor, an acid catalystmay be used. In this case, the sol is a viscous liquid that can used formaking or applying the coating by rolling, screen printing, or by use ofa brush or other mechanical implements to spread the sol evenly on thesurface of a substrate.

The coating mixture may also optionally include other components toimprove performance of the final coating or to improve stability andshelf life of the sol mixture prior to its application on a substrate.These additional components may include low molecular weight polymers,particle size tuning additives, cross-linking additives, and surfacemodifying additives. It should be appreciated that these optionaladditives may be used separately or in any combination in a coatingmixture. Further, it should be appreciated that these additionalcomponents can be used in any combination with any of the silaneprecursors and with any combination of solvents and acids or basesdescribed above.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a low molecular weight polymer. The polymer functionsas a binder for mechanical stability of the coatings. It also helps withthe uniform spreading of the sol liquid on a substrate to provide ahomogeneous coating. In some embodiments, depending upon the intendeduse of the final coating, the amount of the polymer can vary from 0.1%to about 10%, from about 0.1% to about 1%, and from about 0.2% to about0.5%, by weight of the of the coating mixture from which the sol ismade. Examples of polymers that may be used includepoly(3,3,3-trifluoropropylmethylsiloxane),tridecafluorooctylmethylsiloxane, and dimethylsiloxane copolymer, aswell as combinations of these polymers. It also should be appreciatedthat combinations of polymers and acid catalysts may be used.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a particle size tuning additive. As the refractiveindex (“RI”) of the coating affects the light transmission through thecoating or the anti-reflective behavior of the coating, components canbe added to the coating mixture to tune the particle size distributionin the sol to adjust the refractive index of the coating to increaselight transmission through the coating, thereby enhancing theanti-reflective ability of the coating.

Non-limiting examples of the particle size tuning additive includesodium acetate, tris(hydroxymethyl)aminomethane (“TRIS”),N-(2-acetamido)iminodiacetic acid,(4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) (“HEPES”),2-(N-morpholino)ethanesulfonic acid (“MES”), imidazole, propanolamine,ethylenediamine, diethylenetriamine, and 3-aminopropyltrimethoxysilane.In some embodiments, the use of a particle size tuning additiveincreases the light transmittance from about 0.5% to about 2.0% comparedto a coating without a particle size tuning additive. The amount ofparticle size tuning additive used may range from about 0.01% to about10% by weight of the coating mixture. In some embodiments, the range isfrom about 0.1% to about 2% by weight of the coating mixture. Example 1below describes the use of a particle size tuning additive.

It should be appreciated that the anti-reflective property of the finalcoating is determined by both the coating composition and the coatingthickness. A particularly surprising and advantageous feature of using aparticle size tuning additive is that in addition to enhancing theanti-reflective properties of the coatings, it also enhances thespreadability of the sol, which leads to the formation of better qualityfilms. The hydrogen bonding groups on these particle size tuningadditives are effective at stabilizing sol particles via prevention ofinter-particle aggregation. Because of their hydrogen bonding propertiesthey alter the viscosity of the sol and make it suitable to form filmswith the desired thickness, which is in the range of 100-150 nm in someembodiments. Because of their polar and hydrophilic nature they alsoalter the interactions of the sol with glass substrates and, therefore,lower the surface tension of the sol to enable better spreading. Theparticle size tuning additives also hydrogen bond with solvent moleculesand, therefore, slow the rate of evaporation to enable uniform andhomogeneous formation of coatings.

In some embodiments, the coating mixture from which the sol is made mayoptionally include a cross-linking additive to improve the abrasionresistance of the final coating. The abrasion resistance of theanti-reflecting coating is necessary for overall stability of thecoating and for long term performance. The abrasion resistance of acoating relates to its ability to withstand the action of externalmechanical forces that can lead to nicks and scratches, indentations,peeling, and associated loss of material from the coating. At a bulklevel the abrasion resistance relates to hardness and/or stiffness ofthe coating, which in turn relates to the ability of the sol to form anextended network via cross-linking. The abrasion resistance of the finalcoating, therefore, can be enhanced by means of cross-linking additivesthat can increase the interactions between the particles by formingcovalent linkages.

These optional cross-linking additives include silane coupling agents ofthe type α,ω-bis(trialkoxysilane). Specific examples of cross-linkingagents from this category include bis(trimethoxysilyl)methane,bis(trimethoxysilyl)ethane, bis(trimethoxysilyl)hexane,bis(trimethoxysilyl)octane, bis(trimethoxysilylethyl)benzene,bis[(3-methyldimethoxysilyl)propyl]propylene oxide. Optionally, thecross-linking agent can be of the trifunctional type such astris(3-trimethoxysilylpropyl)isocyanurate. Without being bound bytheory, it is believed that these additives when added to the coatingmixture increase cross-linking between particles and enhance themechanical properties of the coatings.

These cross-linking additives can be added to the coating mixture fromwhich the sol is formed or, in the case where separate sols are combinedto form a final sol, the cross-linking additives can be added to thatfinal sol. The amount of cross-linking additive used depends upon theamount of silane precursors used and is relative to the total amount ofall the silane precursors used. The cross-linking additive to totalsilane ratio can vary from about 0.1 to about 1. In some embodiments,the cross-linking additive to total silane ratio is from about 0.2 toabout 0.8. For trifunctional silane cross-linking additives, thecross-linking additive to total silane ratio is on the lower side ofthese ranges, while for bi-functional cross-linking additives, thecross-linking additive to total silane ratio it is on the higher side.Example 2 below describes the use of a cross-linking additive.

The coating composition may also optionally comprise surface modifyingadditives. Typically, the surface modifying additives would be usedinstead of a cross-linking additive. The surface modifying additivescomprise a two component system with complementary functional groupsthat can react with each other to form a covalent bond. The surfacemodification of the coating is carried out by a two step method. First,an active precursor with cross-linkable functional groups (e.g., asilane coupling agent with reactive functional groups) is incorporatedinto the sol mixture to form a coating containing the functional groups.Second, the coating is treated with a hydrophobic reactive agent thatcan react with the embedded functional groups. This second step isperformed after the sol has gelled and as any solvent evaporates. Insome embodiments, this may take from about 30 seconds to about 2minutes. Once the gel sets, the gel is immersed in the hydrophobicreactive agent to initiate a reaction between the active precursor withcross-linkable functional groups (e.g., a silane coupling agent withreactive functional groups) and the hydrophobic reactive agent.Therefore, the active precursor with cross-linkable functional groups isincorporated into the coating followed by treatment of the coating withthe hydrophobic reactive agent that can react with the functional groupsin the coating to form a surface layer with hydrophobic groups on thesurface.

The active precursor with cross-linkable functional groups may be asilane coupling agent incorporated into the sol that has functionalgroups such as alcohol or silanol (—OH) aldehyde (—CHO), or isocyanate(—NCO) while the hydrophobic reactive agent has amine (—NH or —NH₂groups), silanol (—OH), or alkoxysilane groups. The silane couplingagent may be alkoxysilane precursors with functional groups that canreact with the hydrophobic reactive agent. Nonlimiting examples of thesilane coupling agent include triethoxysilyl butyraldehyde,hydroxymethyltriethoxysilane, 3-isocyanatopropyltriethoxysilane,3-isocyanatopropyltrimethoxysilane, isocyanatomethylldimethoxysilane,and isocyanatomethylldiethoxysilane. Examples of the hydrophobicreactive agent include hexamethyldisilazane, hexamethylcyclotrisilazane,and dichlorohexamethyltrisiloxane. The amount of active precursor withcross-linkable functional groups used depends upon the total silaneprecursor present in the coating mixture. The active precursor to totalsilane precursor ratio can vary from about 0.1 to about 1. In someembodiments, the active precursor to total silane precursor ratio variesfrom about 0.2 to about 0.8.

Since the hydrophobic reactive agent is applied to the gel state of thecoating mixture, the reactions that occur take place nominally on thesurface of the gel. Therefore, the hydrophobic reactive agent is notapplied in stoichiometric proportions. Typically, an about 1% to about5% by weight solution of isopropanol (“IPA”) is applied to the gelsurface. In some embodiments, an about 1% to about 2% by weight solutionof IPA is applied to the gel surface.

Example 4 below describes the use of a surface modifying additive. Itshould be appreciated that in some embodiments, the surface modificationof the coating retains the optical and mechanical properties of thecoating but increases surface contact angle by up to 15 degrees, whichimproves the anti-soiling or self-cleaning properties of the coating.

Table 1 lists various coating mixtures according to various embodimentsof the present invention.

TABLE 1 Fluorosilane Organosilane Solvent¹ Acid Polymer² (vol. %) (vol.%) (vol. %) (vol. %) (vol. %) (tridecafluoro-1,1,2,2-tetrahydrooctyl)tri- ethoxysilane (100%) (tridecafluoro-1,1,2,2-tetrahydrooctyl)di- methylchlorosilane (100%)(3,3,3-trifluoropropyl)tri- methoxysilane (100%)(3,3,3-trifluoropropyl)tri- chlorosilane (100%) (3,3,3-trifluoropropyl)methyldi- methoxysilane (100%) methyltrichlorosilane(100%) methyltrimethoxysilane (100%) phenylmethyldi- ethoxysilane (100%)(tridecafluoro-1,1,2,2- I (50%) tetrahydrooctyl)tri- ethoxysilane (50%)(tridecafluoro-1,1,2,2- I (50%) tetrahydrooctyl)di- methylchlorosilane(50%) (3,3,3-trifluoropropyl)tri- I (50%) methoxysilane (50%) (3,3,3- I(50%) trifluoropropyl)methyldi- methoxysilane (50%)(3,3,3-trifluoropropyl)tri- I (50%) chlorosilane (50%)methyltrichlorosilane I (50%) (50%) methyltrimethoxysilane I (50%) (50%)(tridecafluoro-1,1,2,2- methyltrimethoxysilane I (33%)tetrahydrooctyl)tri- (33%) ethoxysilane (33%) (tridecafluoro-1,1,2,2-dimethyldimethoxysilane I (33%) tetrahydrooctyl)tri- (33%) ethoxysilane(33%) (tridecafluoro-1,1,2,2- n-propylmethyldichloro- I (33%)tetrahydrooctyl)tri- silane (33%) ethoxysilane (33%) (3,3,3-methyltrimethoxysilane I (33%) trifluoropropyl)methyldi- (33%)methoxysilane (33%) (3,3,3-trifluoropropyl)tri- methyltrimethoxysilane I(33%) methoxysilane (100%) (33%) (tridecafluoro-1,1,2,2-methyltrimethoxysilane I (32%) 0.04 m tetrahydrooctyl)tri- (33%) HClethoxysilane (33%) (1%) (3,3,3-trifluoropropyl)tri-methyltrimethoxysilane I (32%) 0.04 m methoxysilane (33%) (33%) HCl (1%)(tridecafluoro-1,1,2,2- dimethyldimethoxysilane I (32%) 0.04 mtetrahydrooctyl)tri- (33%) HCl ethoxysilane (33%) (1%)(tridecafluoro-1,1,2,2- methyltrimethoxysilane I (32%) A (1%)tetrahydrooctyl)tri- (33%) ethoxysilane (33%) (tridecafluoro-1,1,2,2-dimethyldimethoxysilane I (32%) A (1%) tetrahydrooctyl)tri- (33%)ethoxysilane (33%) (3,3,3-trifluoropropyl)tri- methyltrimethoxysilane I(32%) A (1%) methoxysilane (33%) (33%) (tridecafluoro-1,1,2,2-dimethyldimethoxysilane I (32%) B (1%) tetrahydrooctyl)tri- (33%)ethoxysilane (33%) (tridecafluoro-1,1,2,2- methyltrimethoxysilane I(32%) B (1%) tetrahydrooctyl)tri- (33%) ethoxysilane (33%)(3,3,3-trifluoropropyl)tri- methyltrimethoxysilane I (32%) B (1%)methoxysilane (33%) (33%) ¹“I” is isopropanol ²“A” ispoly(3,3,3-trifluoropropylmethylsiloxane); “B” is a copolymer of 15-20%tridecafluorooctylmethylsiloxane and 80-85% dimethylsiloxane

Other specific coating compositions according to various embodiments ofthe present invention include the following: (1) 0.38%tetramethoxysilane, 0.47% methyl trimethoxysilane, 0.56% trifluoropropyltrimethoxysilane, 0.05% HCl, 12.17% water, and 86.37% isopropanol; (2)0.38% tetramethoxysilane, 0.47% methyl trimethoxysilane, 0.56%trifluoropropyl trimethoxysilane, 0.04% NH₄OH, 12.18% water, and 86.37%isopropanol; and (3) 0.34% tetramethoxysilane, 0.47% methyltrimethoxysilane, 0.56% trifluoropropyl trimethoxysilane, 0.05% HCl,12.18% water, and 86.40% isopropanol, where all percentages are weightpercents.

The methods for preparing or formulating the coating compositions ormixtures from which the final sol is prepared will now be described. Asnoted above, the composition of the coating composition is based on useof a silane precursor or mixture of silane precursors. In those cases inwhich only one silane precursor is used, the process for preparing thesol basically includes mixing the silane precursor and any solventfollowed by the addition of any catalyst and then any other components.In those cases in which multiple silane precursors are used, the silaneprecursors can be mixed and hydrolyzed together or they can behydrolyzed separately and then mixed together prior to coating asubstrate. The strategy of hydrolyzing the precursors separately makesit possible to control the amounts of each precursor and also preventsextensive aggregation between the silanol species to maintain the lowviscosity necessary for depositing thin films. It should be appreciatedthat it is also possible to hydrolyze some of the silane precursorstogether and hydrolyze one or more other silane precursors separatelyand then mix all of them together prior to coating a substrate.

For example, the compositions listed in Table 1 above are based on aone-pot reaction or batch process. The components are mixed together toform the liquid or sol that is deposited onto the substrate. The orderof mixing includes mixing the solvent and silane precursor(s), thenadding acid (if any) and then polymer (if any). In other embodiments inwhich only one silane precursor is used, the method involves adding thesilane precursor, then the acid or base catalyst, and then water,followed by sonication. For other coating compositions, including thosedescribed in Examples 1, 2, and 3 below and those that use multiplesilane precursors, the silanes can be hydrolyzed separately and thenmixed together, or alternatively, they can be hydrolyzed together as aone-pot reaction system or batch process. In the latter case, the silaneprecursor(s) are added to the solvent followed by the addition of water.Then any acid or base catalyst is added followed by sonication. Itshould be appreciated that either a one-pot reaction to form the sol orformation of separate sols can be used; however, if multiple silaneprecursors are used, separate sol formation following by mixing providesbetter control over formation of the final mixed sol.

In one embodiment in which three silane precursors are used, a typicalmethod of preparation involves hydrolysis of two of the silaneprecursors, such as an organosilane and an organofluorosilane,separately in isopropanol under acidic (or basic) conditions. In thiscase, each of the two silane precursors is separately mixed with asolvent and any acid or base in the amounts described above. Dependingupon the total quantity being processed, mixing can be done by anymethod known in the art, including shaking, swirling, or agitation underambient conditions. For larger volumes a larger scale mixer may be used.

After mixing, each of the two individual sols is prepared by hydrolyzingthe respective silane precursor in an ultrasonic bath maintained atambient conditions for 30 minutes. In other words, the first mixture oforganosilane, solvent, and acid/base is placed in an ultrasonic bath,and the second mixture of organofluorosilane, solvent, and acid/base isseparately placed in an ultrasonic bath. Sonication is used to enablemixing at a molecular level for the hydrolysis reaction to take placebetween water and the silane precursor, which are immiscible. Avariation of the process involves hydrolysis in an ultrasonic bath at anelevated temperature to accelerate the hydrolysis process. In someembodiments, the temperature may be in the range of from about 40° C. toabout 80° C. When hydrolyzed, these silane precursors form a particulatesuspension of particles in a liquid or sol.

After being hydrolyzed, the two sols (e.g., one sol containinghydrolyzed organosilane and one sol containing hydrolyzedorganofluorosilane) are mixed together along with a third silaneprecursor, for example, a tetraalkoxysilane, and the entire mixture isfurther sonicated for 30 minutes in the ultrasonic bath. The combined orfinal sol is then allowed to age under ambient conditions for a variableamount of time from 30 minutes to 48 hours. Aging is required for thesol-gel reactions (hydrolysis and condensation) to go to completion andfor the sol to obtain a steady state condition. Aging time alsofacilitates reaction between particles from the different sols to form ahomogeneous mixture. The relative extent of aging dictates sol viscosityand consequently film homogeneity and film thickness, which in turn,controls the anti-reflective properties of the coating. In someembodiments, longer aging times of 24-48 hours may be used since theyallow sufficient time for the system to reach equilibrium. Shorter agingtimes may be used to prevent formation of bigger particles to obtain atargeted viscosity and coating thickness upon application of the sol toa substrate for sol systems that are too reactive or too concentrated.

Variations of this method may be made in some embodiments. For example,the final sol can be made by mixing all of the silane precursorsconcurrently in one container and hydrolyzing them together in onereaction. Alternatively, when using three silane precursors (e.g., anorganosilane, an organofluorosilane, and a tetraalkylsilane) threeseparate sols can be formed and then mixed together to form the finalsol, as opposed to forming two sols and then mixing these with a thirdsilane precursor.

Also, the sonication times can vary from 30 minutes to 5 hours and canbe done at room temperature conditions or can be done at elevatedtemperatures of up to 80° C. to facilitate to completion the sol-gelreactions of hydrolysis and condensation. Longer sonication times arepreferred since they ensure that the reactions have reached asteady-state equilibrium. Increasing temperature also helps thereactions to go to completion, which permit the use of shortersonication times. Increased temperature may also assist in formingsmaller particles, since the reaction rates may be significantlyenhanced at higher temperature to form a large number of smallerparticles as opposed to a smaller number of large particles. In someembodiments, small particles in the range of 1-10 nm are preferred forforming coatings with optimum anti-reflection properties. Also, as notedthe aging time for the sol can vary from 30 minutes to 48 hours to allowsufficient time for the system to reach equilibrium and to produce thedesired particle size and viscosity. In other words, the sonication timeand temperature and the aging time can be adjusted to produce anequilibrated sol having the desired particle size, which in someembodiments is particles having a size of about 1 nm to about 10 nm, andviscosity.

The application of the coating mixtures above will now be described. Thecoating mixtures described herein are used to form a uniform,homogenous, optical quality, crack-free coating that is largely devoidof defects and imperfections. The methods of fabricating a durablecoating are based on a combination of factors including the compositionof the coating mixture, the use of appropriate solvents or combinationof solvents, substrate preparation, coating methods, and specific curingconditions.

In some embodiments, for example, the coating mixtures described inTable 1, the coating mixtures of the present invention may be in theform of a stable liquid composition that can be applied to a givensubstrate. The coating mixture that is applied to a substrate may alsobe in the form of a gel, lotion, paste, spray, or foam. In someembodiments, the coating mixture that is applied to a substrate is inthe form of a liquid or viscous fluid or clear liquid. In someembodiments, the coating compositions are present as a clear liquid foruse as a spray, or alternatively, as a dispersion, viscous liquid, or amixture of these. In some embodiments, the coating mixtures have aviscosity in the range of approximately 0.5-5 cP for clear liquidcompositions and approximately 10-200 cP in the form of pre-hydrolyzedviscous liquid, where pre-hydrolyzed refers to a liquid sol containinghydrolyzed particles or hydrolyzed silane precursors. It should beappreciated that in some cases, the coating mixture can be applied tothe substrate before hydrolysis. In other words, the coating mixture isapplied to the substrate after which the coating mixture is hydrolyzedand condensed with the aid of moisture in the air. Accordingly, the solis formed on the substrate followed by formation of the final coating.It should be appreciated that the state of the coating mixture forapplication on a substrate can be determined based upon the form mostapplicable to form a thin film or coating in conjunction with themechanical or physical method used for actually applying the coatingmixture.

It should be appreciated that the coating material and process by whichit is applied to the substrate comprise a larger coating system. Thecoating material is optimized for a particular coating method and viceversa. Thus the optimized coating process is preferentially performed bya custom designed tool to insure consistency and quality. Therefore,this tool coupled with the coating materials comprises the coatingsystem. Given that the benefits of the current invention areparticularly well suited to solar panel manufacturers, who do notthemselves manufacture tools, it is desirable to offer a completesolution including both the coating material and its associated coatingtool. In the following paragraphs describing the coating process itshould be appreciated that these steps could be executed manually,automatically using a coating tool, or in any combination of both.

Furthermore, the custom designed coating tool may be a large stand-aloneunit intended for operation in a factory setting. It could also be asub-tool, such that it comprises a process module that performs thecoating process but that is integrated into another machine thatperforms other steps in the larger solar panel manufacturing process.For example, it could be a module attached to an existing glass washingmachine or a module attached to a panel assembly machine. Alternatively,the tool could be portable or semi-portable, for example, mounted on atruck or inside a tractor trailer such that it could be transported to aworksite and used to coat solar modules during the construction of alarge solar installation. Alternatively it could be designed such thatthe coating could be applied to installed solar modules in-situ.

In general, three steps are used to apply the sol to a given substrate.First, the substrate is cleaned and prepared. Second, the substrate iscoated with the sol or mixture of sols. Third, the final coating isformed on the substrate.

As an initial step, the substrate is pre-treated or pre-cleaned toremove surface impurities and to activate the surface by generating afresh surface or new binding sites on the surface. The substratepre-treatment steps are important in providing uniform spreading anddeposition of the sol, effective bonding interactions between thesubstrate and coating material for Si—O—Si linkage formation, andprevention of defects and imperfections at the coating-substrateinterface because of uneven spreading and/or diminished bondinginteractions due to surface inhomogeneities.

In particular, it is desirable to expose Si—OH groups on the surface ofthe substrate through pre-treatment or cleaning of the substrate surfaceto form an “activated” surface. An activated surface layer lowers thesurface tension of the predominantly hydrophilic solvents in the sol andenables effective spreading of the sol on the surface. In someembodiments, a combination of physical polishing or cleaning and/orchemical etching is sufficient to provide even spreading of the sol. Incases, where the surface tension would need to be further lowered, thesubstrate, such as glass, may be pretreated with a dilute surfactantsolution (low molecular weight surfactants such as surfynol; long chainalcohols such as hexanol or octanol; low molecular weight ethylene oxideor propylene oxide; or a commercial dishwasher detergent such asCASCADE, FINISH, or ELECTRASOL) to further help the sol spread better onthe glass surface.

Accordingly, surface preparation involves a combination of chemical andphysical treatment of the surface. The chemical treatment steps include(1) cleaning the surface with a solvent or combination of solvents, (2)cleaning the surface with a solvent along with an abrasive pad, (3)optionally chemically etching the surface, and (4) washing the surfacewith water. The physical treatment steps include (1) cleaning thesurface with a solvent or combination of solvents, (2) cleaning thesurface with a solvent along with particulate abrasives, and (3) washingthe surface with water. It should be appreciated that a substrate can bepre-treated by using only the chemical treatment steps or only thephysical treatment steps. Alternatively, both chemical and physicaltreatment steps could be used in any combination. It should be furtherappreciated that the physical cleaning action of friction between acleaning brush or pad and the surface is an important aspect of thesurface preparation.

In the first chemical treatment step, the surface is treated with asolvent or combination of solvents with variable hydrophobicity. Typicalsolvents used are water, ethanol, isopropanol, acetone, and methyl ethylketone. A commercial glass cleaner (e.g., WINDEX) can also be employedfor this purposes. The surface may be treated with an individual solventseparately or by using a mixture of solvents. In the second step, anabrasive pad (e.g., SCOTCHBRITE) is rubbed over the surface with the useof a solvent, noting that this may be performed in conjunction with thefirst step or separately after the first step. In the last step, thesurface is washed or rinsed with water.

One example of substrate preparation by this method involves cleaningthe surface with an organic solvent such as ethanol, isopropanol, oracetone to remove organic surface impurities, dirt, dust, and/or grease(with or without an abrasive pad) followed by cleaning the surface withwater. Another example involves cleaning the surface with methyl ethylketone (with or without an abrasive pad) followed by washing the surfacewith water. Another example is based on using a 1:1 mixture of ethanoland acetone to remove organic impurities followed by washing the surfacewith water.

In some instances an additional, optional step of chemically etching thesurface by means of concentrated nitric acid, sulfuric acid, or piranhasolution (1:1 mixture of 96% sulfuric acid and 30% H₂O₂) may benecessary to make the surface suitable for bonding to the deposited sol.Typically this step would be performed prior the last step of rinsingthe surface with water. In one embodiment, the substrate may be placedin piranha solution for 20 minutes followed by soaking in deionizedwater for 5 minutes. The substrate may then be transferred to anothercontainer holding fresh deionized water and soaked for another 5minutes. Finally, the substrate is rinsed with deionized water andair-dried.

The substrate may alternatively or additionally prepared by physicaltreatment. In the physical treatment case, for one embodiment thesurface is simply cleaned with a solvent and the mechanical action of acleaning brush or pad, optionally a surfactant or detergent can be addedto the solvent, after which the substrate is rinsed with water and airdried. In another embodiment, the surface is first cleaned with waterfollowed by addition of powdered abrasive particles such as ceria,titania, zirconia, alumina, aluminum silicate, silica, magnesiumhydroxide, aluminum hydroxide particles, or combinations thereof ontothe surface of the substrate to form a thick slurry or paste on thesurface. The abrasive media can be in the form a powder or it can be inthe form of slurry, dispersion, suspension, emulsion, or paste. Theparticle size of the abrasives can vary from 0.1 to 10 microns and insome embodiments from 1 to 5 microns. The substrate may be polished withthe abrasive slurry via rubbing with a pad (e.g., a SCOTCHBRITE pad), acloth, or paper pad. Alternatively, the substrate may be polished byplacement on the rotating disc of a polisher followed by application ofabrasive slurry on the surface and rubbing with a pad as the substraterotates on the disc. Another alternative method involves use of anelectronic polisher that can be used as a rubbing pad in combinationwith abrasive slurry to polish the surface. The substrates polished withthe slurry are cleaned by pressurized water jet and air-dried.

After pretreating the surface, the final sol is deposited on a substrateby techniques known in the art, including dip coating, spraying, droprolling, or flow coating to form a uniform coating on the substrate.Other methods for deposition that can be used include spin-coating;aerosol deposition; ultrasound, heat, or electrical deposition means;micro-deposition techniques such as ink-jet, spay-jet, xerography; orcommercial printing techniques such as silk printing, dot matrixprinting, etc. Deposition of the sol is typically done under ambientconditions.

In some embodiments, the method of deposition is performed via the droprolling method on small surfaces wherein the sol composition is placedonto the surface of a substrate followed by tilting the substrate toenable the liquid to roll across the entire surface. For largersurfaces, the sol may be deposited by flow coating wherein the sol isdispensed from a single nozzle onto a moving substrate at a rate suchthat the flowing sol leads to a uniform deposition onto a surface orfrom multiple nozzles onto a stationary surface or from a slot onto astationary surface. Another method of deposition is via depositing theliquid sol onto a substrate followed by use of a mechanical dispersantto spread the liquid evenly onto a substrate. For example, a squeegee orother mechanical device having a sharp, well-defined, uniform edge maybe used to spread the sol.

In addition to the actual methods or techniques used to deposit thefinal sol on the substrate, it should be appreciated that severalvariations for depositing the final sol exist. For example, in oneembodiment, the final sol is simply deposited on the substrate in onelayer. In another embodiment, a single sol or multiple sols may bedeposited to form multiple layers, thereby ultimately forming amultilayered coating. For example, a coating of the sol containing anorganosilane and an organofluorosilane can be formed as an underlayerfollowed by a topcoat of a tetraalkoxysilane. In another embodiment, anunderlayer of an organosilane may be deposited followed by thedeposition of a topcoat of a mixture of an organofluorosilane and atetraalkoxysilane. In another embodiment, an underlayer of atetraalkoxysilane may be deposited followed by the deposition of a toplayer using a sol mixture of an organosilane and an organofluorosilane.In another embodiment, an underlayer of a sol made from a mixture of anorganosilane and an organofluorosilane may be deposited followed byvapor deposition of a top layer by exposing the layer to vapors of atetraalkoxysilane. In another embodiment, an underlayer of a sol madefrom a mixture of an organosilane and an organofluorosilane may bedeposited followed by deposition of a top layer by immersing thesubstrate in a solution of a tetraalkoxysilane in isopropanol. In theembodiments in which multiple layers are deposited, each layer may bedeposited almost immediately after deposition of the first layer, forexample, within or after 30 seconds of deposition of the prior layer. Asnoted, different sols may be deposited on top of one another, ordifferent mixtures of sols may be deposited on top of one another.Alternatively, a single sol may be deposited in multiple layers or thesame sol mixture may be deposited in multiple layers. Further, a givensol may be deposited as one layer and a different sol mixture may beused as another layers. Further, it should be appreciated that anycombination of sols may be deposited in any order, thereby constructinga variety of multi-layered coatings. Further, it should be appreciatedthat the sols for each layer may be deposited using different techniquesif so desired.

The thickness of the coatings deposited can vary from about 10 nm toabout 5 micron. In some embodiments, the thickness of the coating variesfrom about 100 nm to about 1 micron, and in other embodiments it variesfrom about 100 nm to about 500 nm. In order to provide sufficientanti-reflective properties, a thickness of about 60 nm to about 150 nmis desired. It should be appreciated that the thickness of the coatingmixture as deposited is affected by the coating method, as well as theviscosity of the coating mixture. Accordingly, the coating method shouldbe selected so that the desired coating thickness is achieved for anygiven coating mixture. Further, in those embodiments, in which multiplelayers of sols are deposited, each layer should be deposited in athickness such that the total thickness of the coating is appropriate.Accordingly, in some embodiments in which multiple layers of sols aredeposited, the overall coating thickness varies from about 100 nm toabout 500 nm, and in order to provide sufficient anti-reflectiveproperties, a total coating thickness of about 60 nm to about 150 nm isdesired.

Once the final sol is deposited as described above, the deposited solwill proceed to form a gel through the process of gelation after whichthe gel is dried and cured to remove residual solvent and facilitatenetwork formation via Si—O—Si linkage formation in the coating. Inaddition, the gel may be allowed to age to allow for the formation ofadditional linkages through continued hydrolysis and condensationreactions.

As described above, the sol-gel method used in preparing the coatingsdescribed herein utilizes a suitable molecular precursor that ishydrolyzed to generate a solid-state polymeric oxide network. Initialhydrolysis of the precursor generates a liquid sol, which ultimatelyturns to a solid, porous gel. Drying of the gels under ambientconditions (or at elevated temperature) leads to evaporation of thesolvent phase to form a cross-linked film. Accordingly, throughout theprocess, the coating mixture/sol/gel/dried/cured coating undergoeschanges in physical, chemical, and structural parameters thatintrinsically alter the material properties of the final coating. Ingeneral, the changes throughout the sol-gel transformation can beloosely divided into three interdependent aspects of physical, chemical,and structural changes that result in altered structural composition,morphology, and microstructure. The chemical composition, physicalstate, and overall molecular structure of the sol and the gel aresignificantly different such that the materials in the two states areintrinsically distinct.

Regarding physical differences, the sol is a collection of dispersedparticles suspended in a liquid. These particles are surrounded by asolvent shell and do not interact with each other significantly. Assuch, the sol is characterized by fluidity and exists in a liquid state.In contrast, in a gel film the network formation has occurred to anadvanced state such the particles are interconnected to each other. Theincreased network formation and cross-linking makes the gel networkrigid with a characteristic solid state. The ability of the material toexist in two different states is because of the chemical changes thatoccur along the sol to gel transformation.

Regarding chemical changes, during the sol to gel transition, the solparticles combine with each other via formation of Si—O—Si linkages. Asa result, the material exhibits network formation and strengthening.Overall, the sol particles contain reactive hydroxyl groups on thesurfaces that can participate in network formation while the gelstructure has these hydroxyl groups converted into siloxane groups.

Regarding structural differences, the sol contains discrete particlescontaining few siloxane linkages along with terminal hydroxyl as well asunhydrolyzed alkoxy ligands. As such, the sol state can be consideredstructurally different from the solidified films which contain majoritysiloxanes. As such, the liquid sol and the solid state polymericnetworks are chemically and structurally distinct systems.

Regarding differences in properties, the origin of the physical andchemical properties of the sol and gel films depends upon theirstructure. The sol particles and the gel films differ in the chemicalcomposition, makeup and functional groups and as a result exhibitdifferent physical and chemical properties. The sol stage because of itsparticulate nature is characterized by high reactivity to form thenetwork while the gel state is largely unreactive due to conversion ofreactive hydroxyl groups to stable siloxane linkages. Accordingly, itshould be appreciated that it is the particular combination of silaneprecursors and other chemicals added to the coating mixture that ishydrolyzed and condensed, gelled, and dried and cured on a substratesurface that gives the final coatings of the present invention thedesired properties described above.

There are several methods by which the gel is dried and cured and/oraged to form the final coating. In some embodiments the gel is dried andcured under ambient or room temperature conditions. In some embodiments,the gel is aged under ambient conditions for 30 minutes followed bydrying for 3 hours in an oven kept at a variable relative humidity of(e.g., 20% to 50%). The temperature of the oven is then increased slowlyat a rate of 5° C./min to a final temperature of 120° C. The slowheating rate along with the moisture slows the rate of the silanolcondensation reaction to provide a more uniform and mechanically stablecoating. This method provides reproducible results and is a reliablemethod of making the coating with the desired properties.

In another embodiment, the gel on the substrate is heated under aninfrared lamp or array of lamps. These lamps are placed in closeproximity to the substrate's coated surface such that the surface isevenly illuminated. The lamps are chosen for maximum emission in themid-infrared region of approximately 3 um to approximately 5 umwavelength. This region is desirable because it is adsorbed better byglass than shorter infrared wavelengths. The power output of the lampsmay be closely controlled via a closed loop PID controller to achieve aprecise and controllable temperature profile. In some embodiments thisprofile will start from ambient temperature and quickly riseapproximately 1 to approximately 50° C. per second to a temperature ofapproximately 120° C., hold that temperature for a period ofapproximately 30 to approximately 300 seconds, then reduce temperatureback to ambient, with or without the aid of cooling airflow.

In another embodiment, the coated substrate is heated on a hot plate at120° C. such that the uncoated surface is in contact with the hot platewhile the coated surface is exposed to air. In this case, the hot plateis turned on to a set temperature of 120° C. after the substrate hasbeen placed on the hot plate. This ensures a slow heating rate toprevent cracking, flaking, and/or delamination of the coating material.

In another embodiment, the coating on the substrate is heated on a hotplate at 200-300° C. wherein the surface of the hot plate is coveredwith a silicone mat that is in direct contact with the surface of thecoating to reach a desired curing temperature of 120° C. within 10-30seconds. The conductive nature of the silicone mat makes it conducive totransfer heat to the coated surface to increase the continuinghydrolysis and condensation reactions. In this case, the coated surfacecan reach a temperature of 120° C. within 10-30 seconds.

It is particularly noteworthy that the coatings of this invention areprepared under temperatures not exceeding 120° C. in contrast totemperatures of 400-600° C. typically employed in curing silica-basedanti-reflective coatings. Another particularly advantageous feature ofsome of the coating compositions herein, particularly those of Table 1,as opposed to the coating mixtures that utilize more than one or morethan two silane precursors, is that they do not require water as aspecific component of the composition for the reaction or curing processto proceed. It is particularly advantageous that the coatingcompositions can be made to harden by reaction with moisture within theenvironment or alternatively by the trace amounts of water present inthe solvent. The curing of the coating in a humid environment slows downthe evaporation of water leading to a coating with improvedcross-linking and better mechanical properties.

As described above and as illustrated further in the Examples, thecoatings made as described herein have several desirable properties. Thecoatings have anti-reflective properties that reduce the reflection ofphotons. The transmittance of a glass substrate coated with a coatingcomposition made according to the present invention can vary from about92% to about 98%, from about 93% to about 96%, and from about 95% toabout 98%.

The coatings also have anti-soiling properties, which are also importantin maintaining sufficient transmittance when used in conjunction with aglass substrate. Soiling is due to adherence of particulate matter onsurfaces exposed to environment. The deposition of the particles ontosurfaces depends upon the surface microstructure as well as chemicalcomposition. In general, rough surfaces can provide many sites forphysical binding of particulate matter. For solar panels, soiling canlead to reduction in power output due to reduced absorption of light oftypically about 5% and in some cases losses of 22% have been reported.The paper “The Effect of Soiling on Large Grid-Connected PhotovoltaicSystems in California and the Southwest Region of the United States”,Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4thWorld Conference, May 2006, Vol. 2, p 2391-2395, reports an average 5%loss. The paper “Soiling and Other Optical Losses in Solar-Tracking PVPlants in Navarra”, Prog. Photovolt: Res. Appl. 2011; 19:211-217,reports losses of 22%.

The chemical composition of the surfaces is reflected in the surfaceenergy as measured by contact angles. Low energy surfaces (characterizedby high water contact angles) are usually less susceptible to binding ascompared to high energy surfaces with low water contact angles.Therefore, anti-soiling properties can be determined indirectly bymeasuring the coating's contact angle. The coatings herein providecontact angles ranging from about 80° to about 178°, from about 110° toabout 155°, and from about 125° to about 175°. The coatings of thisinvention were also characterized for anti-soiling behavior bysubjecting them to repeated cycles of dirt exposure followed by aircleaning (see, for example, FIGS. 9 and 10 and Table 3 below). Thecoatings of this invention minimize the photon flux losses due tosoiling by about 50% relative to uncoated samples.

The coatings of the present invention also provide desirable mechanicalproperties. Nanoindentation is a method of used to measure themechanical properties of nanoscale materials especially thin films andcoatings. In nanoindentation measurements a load is applied to theindenter tip that provides the necessary force for the tip to be pushedinto the sample thereby creating a nanoscale indent on the sample.Nanoindentation measurements involve continuous measurement of the load,contact stiffness, and depth of penetration of the tip with respect totime to record a load-displacement profile. The load displacement curvesin combination with the contact displacement can be used to measuremechanical properties such as hardness and Young's Modulus (or modulusof elasticity). Noting that the typical hardness for a mixture of puresilica sol-gels coatings is observed to be around 1.05 GPa, the coatingsof the present invention exhibit much greater hardness (see, forexample, FIGS. 7 a, 7 b, 8 a and 8 b and Table 3 below). In addition,the pencil hardness of the coating of the present invention can varyfrom about 2H to about 9H, from about 4H to about 7H, and from about 6Hto about 9H.

The coatings of the present invention also provide desirable abrasionresistance. Abrasion resistance can be defined as the ability of amaterial to withstand erosion due to frictional forces to preserve andmaintain its original shape and appearance. Abrasion resistance relatesto the strength of the intrinsic framework structure as well as tosurface features. Materials that do not have sufficient strength due tolack of long range bonding interactions tend to abrade easily.Similarly, materials with uneven surfaces or coatings with surfaceinhomogeneities and asperities tend to wear due to frictional losses.Also, the leveling and smoothening of these asperities due to frictionleads to changes in optical transmission of the coating as the materialis abraded.

The coatings of the present invention pass the standard test formeasuring abrasion resistance of coatings on surfaces as definedaccording to European Standard EN-1096-2 (Glass in Building, CoatedGlass). The test involves the action of rubbing a felt pad on the coatedglass. The felt rubbing pad is subjected to a to-and-fro translationmotion with a stroke length of 120±5 mm at a speed of 54-66strokes/minute combined with a continuous rotation of the pad of 6 rpmor of a rotation of between 10° to 30° at the end of each stroke. Theback and forth motion along with the rotation constitutes 1 cycle. Thespecifications of the circular felt rubbing pad include a diameter of14-15 mm, thickness of 10 mm and density of 0.52 g/cm2. The felt pad isattached to a mechanical finger that is 15 mm to 20 mm is diameter andplaced under a load of 4 Newtons. The transmission at 550 and 900 nm ismeasured to evaluate abrasion resistance and the standard dictates achange in transmission of no more that ±0.05 with respect to a referencesample.

In general, the various coatings of the present invention provide ameans of making a transparent substrate or glass transmit more photonswithout altering its intrinsic structure and other properties, alongwith passivating the surface so that it becomes resistant to theadhesion of water, dirt, soil, and other exogenous matter. Accordingly,the coating mixtures and resulting gels and coatings as described hereinhave varied commercial applications.

Regarding the coating mixtures themselves, these may be sold as acoating mixture or commercial coating formulation for others to use. Forexample, the coating mixtures may be provided as a liquid composition,for example, for subsequent small scale treatment of glass in atreatment separate from their usage as windows in solar or architecturalsystems. In this case the coating mixture may be sold before the silaneprecursors are hydrolyzed. Alternatively, the coating mixtures may besold as sols or after the silane precursors have been hydrolyzed.

In addition, the coating mixtures may be deposited and allowed to gel ona particular substrate that is subsequently sold. In particular, thecoating compositions of the present invention can be coated onto anytransparent substrate that has hydrogen bond donor or hydrogen bondacceptor groups on the surface. For example, the coating can be appliedas a treatment for a given glass or other transparent substrate beforeor after it has been integrated into a device, such a solar cell,optical window or enclosure, for example, as part of a glass treatmentprocess. In other embodiments, the invention provides for the use of thecoating compositions as an efficiency enhancement aid in architecturalwindows in building and houses by the provision of anti-reflectionbenefits and/or by the provision of anti-soiling benefits to augment theanti-reflection benefits. In other embodiments, the invention providesfor the use of the coating compositions as an efficiency enhancement aidin treatment of transparent surfaces that require regular cleaning tomake them self-cleaning. For example, the coatings can be used inconjunction with glass used in windows, windshields, screens,architecture, goggles, eyeglasses, etc.

In other embodiments, the invention provides for the use of the coatingcompositions as an efficiency enhancement aid in photovoltaic solarpanel assemblies (e.g., the outer cover of solar panels) by theprovision of anti-reflection benefits and/or by the provision ofanti-soiling benefits to augment the anti-reflection benefits. Thesedevices convert solar energy into electrical energy and rely uponefficient absorption of photons, and effects such as reflection,scattering, and loss of absorption due to adsorbed soil or dirtparticles can lead to reduced power output. As noted, the coatings ofthis invention when coated onto a glass surface reduces reflection ofphotons (the so-called anti-reflective property) and also reducesadsorption and binding of dirt, soil, and other particulate matter fromthe environment to boost the transmission of photons through the glassas well as to prevent reduction in photons associated with deposition ofparticulate matter onto the surface.

The coatings for solar panel applications provide unique challenges thatare not present with coatings typically utilized in other commonapplications. The use of anti-reflective coating in solar panelsnecessitates long term exposure of solar radiation that usually resultsin extensive degradation of polymeric materials under prolonged UVexposure due to photolytic breakdown of bonds in these materials. Thecoating compositions of the present invention utilize silane precursorsthat when hydrolyzed and dried and cured give rise to a network that issimilar to glass with Si—O—Si bonds that are stable to radiativebreakdown. An additional advantage of using silica based materials insolar applications is the intrinsic hardness of the material that makesthe coating resistant to scratches, indentations, and abrasion. Further,the coatings of the present invention provide for enhanced lighttransmittance across the entire solar region from about 400 nm to about1150 nm, which is desirable for solar applications.

Further, it should be appreciated that the sols resulting from thecoating compositions of this invention do not need to be applied to thesolar panels during manufacturing and may be applied after manufacturingto avoid any interference with the solar panel manufacturing process. Itis expected that the solar panel maker themselves may be able to use thecomposition of this invention to coat the modules at appropriate pointswithin their manufacturing process. In such instances, the provision ofa stable sol, that can be used according to the methods describedherein, provides a direct means for the applying the coating mixtureafter manufacture of the panels or even after final installation of thepanels. This may streamline the manufacturing process and enhance theeconomic value of existing panels, either existing inventory or panelsalready installed and in use, to which the coatings can be applied.

Coating mixtures that can be used specifically for coating solar panelsinclude (1) 0.38% tetramethoxysilane, 0.47% methyl trimethoxysilane,0.56% trifluoropropyl trimethoxysilane, 0.05% HCl, 12.17% water, and86.37% isopropanol; (2) 0.38% tetramethoxysilane, 0.47% methyltrimethoxysilane, 0.56% trifluoropropyl trimethoxysilane, 0.04% NH₄OH,12.18% water, and 86.37% isopropanol; and (3) 0.34% tetramethoxysilane,0.47% methyl trimethoxysilane, 0.56% trifluoropropyl trimethoxysilane,0.05% HCl, 12.18% water, and 86.40% isopropanol, where all percentagesare weight percents.

In one embodiment, the process of coating the solar panels consists ofpreparing the panel surface, coating the surface with the final sol madein accordance with the present invention, drying the coating underambient conditions, and curing the dried panels at elevated temperature.The panel surface is prepared by polishing the panel with a cerium oxideslurry, followed by washing the panel with water, and drying it underambient temperature-pressure conditions for a period ranging from about10 hours to about 12 hours.

Once the panel surface is prepared, in one embodiment, the final sol ofthe present invention is deposited onto solar panels by means of a flowcoater. The sol is deposited onto the panels via gravitational free flowof the liquid sol from top to bottom. The solar panels are placed on themobile platform that moves at a rate that is optimal for the free flowof the sol without introducing break points in the liquid stream orintroducing turbulent flow. The rate of liquid flow and the rate ofmovement of platform carrying the solar panel are optimized fordeposition of uniform, crack-free coatings that are homogenous, free ofdeformities, and characterized by uniform thickness.

More specifically, in one embodiment, the panel is placed on a mobilestage that is connected to a computer and programmed to move at a speedranging, in some embodiments, from about 0.05 cm/s to about 300 cm/s, inother embodiments from about 0.1 cm/s to about 10 cm/s, and in otherembodiments from about 0.25 cm/s to about 0.5 cm/s. The sol is thendeposited onto the panel surface using a nozzle dispensing unit (that isconnected to a computer) such that the rate of flow of sol is, in someembodiments, from about 5 ml/min to about 50 ml/min, in otherembodiments from about 5 ml/min to about 25 ml/min, and in otherembodiments from about 10 ml/min to about 15 ml/min. It should beappreciated that the rate at which the sol is deposited is important forproper deposition of the coatings. Notably, the nozzle diameter of thesol can be adjusted to ensure appropriate flow rate, with diameters ofthe nozzle ranging from about 0.3 mm to about 0.9 mm.

A particularly advantageous aspect of using a sol is that it is in aliquid state but is also viscous enough to spread without breakdown ofthe stream. The uniformity of the coatings is further ensured byadjusting the flow rate and the rate of the movement of the platformcontaining the solar panels. For a given flow rate of the sol, if therate of the movement of the platform is too fast then it leads torupture of the sol stream causing uneven coatings. For a given flow rateof the sol, if the rate of the movement of the platform is too slow itresults in turbulent flow that deteriorates the uniformity of the films.Therefore, a specific optimum of sol flow rate and the platform movementare important to provide even, uniform, and homogenous coatings. The useof specific pH, solvent, and silane concentrations as outlined aboveprovide the ideal viscosities.

The coating process is also facilitated by the evaporation of thesolvent during the flow of the sol onto the panel, which also affectsthe development of uniform films or coatings on the panel surface. Thecoatings are formed when the free flowing sol dries on the surface andforms a solid on the glass surface. More specifically, the bottom edgeof the sol represents the wet line while drying occurs at the top edge.As the solvent evaporates, the sol becomes more viscous and finally setsat the top edge while the bottom edge is characterized by liquid edgespreading. The spreading liquid at the bottom edge enables the free flowof the sol while the setting sol at the top edge fixes the materials andprevents formation of lamellar structures. A balance of these factors isimportant for formation of uniform films.

It should be appreciated that the flow coating method does not allowseepage of the sol into the internal parts of the solar panel assemblyas the excess sol can be collected into a container at the bottom of theassembly and recycled. Similarly, it does not facilitate corrosionand/or leaching of the chemicals from the interior of the solar panelassembly. The flow coater method exposes only the glass side to the solwhile the other side of the panel assembly with electrical contacts andleads does not come into any contact with the liquid sol. As such, theflow coating process is particular beneficial to coating solar panelsduring either the assembly or the post-assembly stages.

The methods described here can be used to coat solar panels of variablesizes and in variable configurations. For example, typical panels havethe dimensions of 1 m×1.6 m, which can be coated either in portraitconfiguration or in landscape mode via appropriate placement the mobileplatform.

The flow coater can be used to coat the panels at the rate of about15-60 panels per hour. The rate of coating of individual panels woulddepend upon the size of the panels and whether they are coated in theportrait mode or landscape orientation. Additionally, multiple coatersoperating in parallel can be used in conjunction with the panel assemblyline to increase the production rate.

After depositing the coating, the panel is dried for a period rangingfrom about 1 minute to about 20 minutes or longer under ambientconditions. The coated panel is then cured using any of the techniquesfor curing described above, after which the coated panel is ready foruse.

The anti-reflecting coatings described herein increase the peak power ofthe solar cells by approximately 3% due to the anti-reflective property.In addition, it is estimated that the anti-soiling property wouldcontribute to minimize transmissive losses associated with accumulationof din on the panels. Typical soiling losses are estimated at about 5%and use of these coatings is expected to reduce the losses in half.

EXAMPLES

The following describes various aspects of the coatings made accordingto certain embodiments of the invention in connection with the Figures.These examples should not be viewed as limiting.

In one embodiment (referred to as “Example 1”), Sol I was prepared byfirst mixing 22.5 mL of isopropanol (“IPA”) and 2.5 mL of 0.04M HCl (pH1.5). 100 μL of methyltrimethoxysilane (“MTMOS”) was then added to thismixture. The final solution of IPA, HCl, and MTMOS was then sonicated ina sonicator for 35 minutes. Sol II was prepared by first mixing 22.5 mLof IPA and 2.5 mL of 0.04M HCl (pH 1.5) followed by adding 100 μL of(3,3,3-trifluoropropyl)-trimethoxysilane (“F3TMOS”). Sol II was alsosonicated for 35 minutes. After sonication, Sol I and Sol II were mixedin equal parts (12.5 mL each), and 100 μL of tetramethoxysilane (“TMOS”)was added. This final solution was then sonicated for 35 minutes. Thismixture was allowed to age under ambient conditions for 24 hours up to120 hours. After aging, microscope slides (polished with cerium oxidepolish, washed, and allowed to dry) were flow coated with the final solmixture and allowed to dry for approximately 5-10 minutes. Once dry, theslides were cured in one of two ways. In one method, the slides wereplaced coated side up on a hot plate/stirrer for 45 minutes at 120° C.In the other method, they were placed in an oven at 25° C. for 25minutes. The temperature was then raised at a constant rate over aperiod of 20 minutes to 120° C., and then maintained at 120° C. for 180minutes. The temperature was then cooled to 25° C. at a constant rateover a period of 60 minutes.

In one embodiment demonstrating the use of a particle size modifyingadditive (referred to as “Example 2”), a sol was prepared usingtris(hydroxymethyl)aminomethane (“TRIS”) as follows: A sol was preparedaccording to the process in Example 1. After final sonication, themixture was allowed to chill in refrigerated water for 5 minutes andthen stand at room temperature for at least 10 minutes. After this, 0.5to 2 mL of 0.5 M solution of TRIS was then added to the mixture, whichwas then shaken and allowed to age for 5 minutes. After aging,microscope slides (polished with cerium oxide polish, washed, andallowed to dry) were then flow coated with the final sol mixture andallowed to dry for approximately 5-10 minutes. Once dry, the slides werecured in either one of two ways. In one method, the slides were placedcoated side up on a hot plate/stirrer for 45 minutes at 120° C. In theother method, they were placed in an oven at 25° C. for 25 minutes. Thetemperature was then raised at a constant rate over a period of 20minutes to 120° C., and then maintained at 120° C. for 180 minutes. Thetemperature was then cooled to 25° C. at a constant rate over a periodof 60 minutes. With these coatings, an anti-reflective enhancement of3.45% at the maximum transmittance and 2.77% averaged over the solarrange was observed.

In one embodiment demonstrating the use of a cross-linking additive(referred to as “Example 3”), a sol was prepared according to theprocess in Example 1. After the final sonication, the mixture wasallowed to chill in refrigerated water for 5 minutes and then stand atroom temperature for at least 10 minutes. After this 1 mL of 0.5 Msolution of TRIS was added to the mixture. After mixing the components,a variable amount (50 μL to 250 μL) of a cross-linker was added. The solis allowed to further age for 5 minutes prior to deposition on thesubstrate.

One variant of Example 3 includes adding a variable amount (50 μL to 250μL) of the cross-linking additive individually to each sol (Sol I andSol II) followed by the same sequence of steps. Another variant ofExample 3 includes mixing Sol I, Sol II, and TMOS along with a variableamount (50 μL to 250 μL) of the cross-linking additive followed bysonication and then adding TRIS to the mixture. Another variant ofExample 3 includes making a multilayer coating with an underlayer madefrom, for example, the coating described above in connection with theuse of TRIS as the particle size tuning additive, followed by depositingan overlayer of 1-5% by volume of cross-linking additive dissolved inIPA.

In one embodiment demonstrating the use of a surface modificationadditive (referred to as “Example 4”), Sol I and Sol II are preparedusing the process described in Example 1. Then equal parts (12.5 mLeach) were mixed, and 100 μL of TMOS was added along with a variableamount (50 μL to 250 μL) of triethoxysilylbutyraldehyde followed bysonication for 35 minutes. The sol was allowed to further age for 5minutes prior to deposition on the substrate. The substrate containingthe coating was further immersed in a solution of hexamethyldisilazane(1% by weight in IPA). The coating was allowed to dry on a hot plate at70° C. for 1 hour to allow the condensation reaction to take placebetween the aldehyde group and amino groups. The substrate containingthe coating was then cured at 120° C. for 45 minutes.

One variant of Example 4 includes forming the coating with the solmixture from Examples 1, 2, or 3 above followed by surface treatment ofthe coating with the silane coupling agent (1% IPA) followed bytreatment with hydrophobic reactive agent (1% IPA). Another variant ofExample 4 includes forming one of the following three sol coatings ((1)0.38% TMOS, 0.47% MTMOS, 0.56% F3TMOS, 0.05% HCl, 12.17% water, and86.37% IPA; (2) 0.38% TMOS, 0.47% MTMOS, 0.56% F3TMOS, 0.04% NH₄OH,12.18% water, and 86.37% IPA; or (3) 0.34% TMOS, 0.47% MTMOS, 0.56%F3TMOS, 0.05% HCl, 12.18% water, and 86.40% IPA, where all percentagesare weight percents) followed by surface treatment with hydrophobicreactive agent (1% solution in IPA).

Where applicable, the measurement of anti-reflective properties of thecoatings was done as follows: The transmittance of the coatings wasmeasured by means of UV-vis absorption spectrophotometer equipped withan integrator accessory. The anti-reflective enhancement factor ismeasured as the relative percent increase in transmittance compared tountreated glass slides versus glass slides coated with compositions ofthis invention. ASTM E424 describes the solar transmission gain, whichis defined as the relative percent difference in transmission of solarradiation before and after the application of the coating. The coatingsexhibit about 1.5% to about 3.25% gain in solar transmission. Therefractive index of the coating was measured by an ellipsometer.

The abrasion resistance of the coating is measured by an abrader deviceaccording to European standard EN-1096-2 (glass in building coatedglass). The coatings made according to Examples 1, 2, and 3, without anyadded composition modifying additives, are able to meet the passingcriteria of the standard.

The contact angle of the coatings is measured by means of goniometerwherein the contact angle of the water droplet is measured by means of aCCD camera. An average of three measurements is used for each sample.

Table 2 presents the results of several performance tests performed oncoatings made according to Example 1. In this Table, the “Spec” refersto the formal procedure for the test performed; the “Pass Criteria”refers to the allowable change in % transmittance (% T) in order for asample to pass the test; “N” is the number of samples/experimentstested; and “Results delta T” is the average of the observed change inpercent transmittance (% T) for the N samples/experiments.

TABLE 2 Pass Result Test Conditions Duration Spec# Criteria N ^(Δ)TAbrasion Resistance 400 g, 14 mm Felt Pad 1000 Strokes EN1096.2 <0.5%  — 0.2% Damp Heat 85° C./85% RH 1000 Hrs IEC61215 <3% 4 1.5% IEC61646Temperature Cycle −40° C. to −85° C. 200 Cycles IEC61215 <3% 4 0.4%IEC61646 Humidity Freeze −40° C. to +85° C., 85% RH 10 Cycles IEC61215<3% 4 0.2% IEC61646 UV Exposure 60° C. 15 kWh/m² (280-400 nm) IEC61345<3% 4 0.7% 7.5 kWh/m² (280-320 nm) Corrosive Atmosphere 1 vol % of SO₂1200 Hrs DIN50018 <1% 5 0.8% UL1332 Salt Spray 5% NaCl in H₂O pH 6.5-7.2at 35° C. 200 Hrs DIN50021 <1% 5 0.5% UL50 Chemical Resistance (Acid) 1MHCl 30 min <0.5%   3 0.3% 1M H₂SO₄ 3 0.4% 1M HNO₃ 3 0.3% ChemicalResistance (Base) 1M NH₄OH 30 min <0.5%   3 0.1% 0.67% aq. (.1675M) NaOH50 min 3 0.2% Boiling Water 100° C. 10 min <0.5%   3 0.3% IndustrialContaminants Wet glass + toner 5 pass with squeegee <1% 3 0.2% wash withwater. Cleaning Tests Common Detergents 1000 brush strokes <1% 3 0.3%Windex 3 0.2%

These results are significant in that they have been achieved with acoating cured at just 120° C. Existing anti-reflective coatings aretypically sintered at approximately 400 to approximately 600° C. toachieve the level of reliability indicated by these results.

FIG. 1 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 2% with coatings on glass slides made fromthe composition given in Example 1.

FIG. 2 illustrates the UV-vis transmittance spectra showing maximumtransmittance enhancement of 3.4% with coatings on glass slides madefrom the composition given in Example 2.

FIG. 3 a is an SEM (cross-sectional view) of a coating made from thecomposition of Example 1 on a glass slide substrate. The SEM images showthe absence of any discernible porosity in these coatings. The filmthickness is about 90 nm.

FIG. 3 b is an SEM (oblique view) of a coating made from the compositionof Example 1 on a glass slide substrate.

FIG. 4 a is an SEM cross-sectional view of a coating made from thecomposition of Example 2 on a glass slide substrate. The SEM images showincreased porosity in these coatings along with presence of surfaceroughness on these coatings. The film thickness is about 100 nm.

FIG. 4 b is a SEM (oblique view) of a coating made from the compositionof Example 2 on a glass substrate.

FIG. 5 shows an XPS spectrum of a coating made from the composition ofExample 1. The peaks indicate presence of fluorine in the coatingscoming from incorporation of (3,3,3-trifluoropropyl)trimethoxysilanealong with silicon and oxygen from silicate network and carbons from theorganic components present in the coating.

FIG. 6 shows an XPS spectrum of a coating made from the composition ofExample 2. The peaks indicate presence of fluorine in the coatingscoming from incorporation of (3,3,3-trifluoropropyl)trimethoxysilanealong with silicon and oxygen from silicate network and carbons from theorganic components present in the coating.

FIGS. 7 a, 7 b, 8 a, and 8 b illustrate nano-indentation data showingthe indenter depth profile, hardness, and Young's Modulus of a coatingmade from the composition of Examples 1 and 2 on a glass slidesubstrate. The results of these measurements are summarized in Table 3.

Table 3 lists various properties for coatings made according to variousembodiments of the present invention, wherein “hardness” is amount offorce (in GPa) exerted by the indenter before the film can exhibitplastic deformations, “contact angle” is the angle of liquid/airinterface at a solid surface or the angle the liquid drop makes with asolid substrate, and “transmission increase” is calculated as {[%T_(coated)−% T_(uncoated)]/[% T_(uncoated)]}×100. It should beappreciated that in some embodiments, the coatings of the presentinvention provide an increase in transmission from about 1% to about3.5% and in some embodiments from about 1.5% to about 3%, and a contactangle of about 80 degrees to about 120 degrees and in some embodimentsabout 85 degrees to about 100 degrees.

TABLE 3 % Average Refractive Coating Young's Transmission Contact IndexThickness Hardness Modulus Coating Increase Angle (ellipsometry)(ellipsometry) (GPa) (GPa) Example 1 1.5% to 2.2% 85-90 degrees 1.4270-100 nm 3.98 62.55 Example 2 2.5% to 2.9% 85-95 degrees 1.31 80-150 nm1.64 50.60

Nano-indentation measurements were performed with a Berkovichnanoindentation system. Typical hardness for a mixture of pure silicasol-gels coatings is observed to be around 1.05 GPa. Without wishing tobe bound by theory, the enhanced mechanical properties of the coatingsof this invention (as compared to pure silica-based coatings) may be dueto several factors that contribute to increased hardness. First, theextensive cross-linking due to the use of the three-precursor systemmakes the Si—O—Si network stronger. Second, the combined use oforganosilane and organofluorosilane enhances the noncovalentinteractions between the organic side chains to promote betterinteractions that enhance the overall mechanical properties. Third, theincreased interactions between the side chains promote a better fillingof porous void space in the sol-gel network to make a homogenous andlargely nonporous coatings. Taken together, the unique combination ofprecursors along with the absence of porous microstructure and theenhanced side chain interactions between the organic groups provides theimproved mechanical properties as compared to coatings of the prior art.

FIG. 9 illustrates the results of accelerated soiling studies showingthe change in solar photon flux passing through the slides, and FIG. 10illustrates the change in weight with respect to sequential exposure ofcoated and uncoated glass slides to simulated dirt storm via wind-drivendeposition of dirt particles. The anti-soiling behavior of the uncoatedand coated slides was measured using the following procedure thatsimulated accelerated soiling of objects that are placed outdoor forextended periods. The absorption spectra of the coated and uncoatedslides were measured to establish their transmittance. The both theslides were weighed. The slides were then placed in a freezer at 4° C.for 5 minutes and exposed to steam from boiling water for 30 seconds tomake their moist with water vapor. These slides were then placed in dustchamber to deposit dust that was evenly dispersed by blowing air from afan. Each slide was exposed to air dispersed with dirt for 3 minutes.The slides were then placed an IR lamp for 1 minute for radiant dryingof the slides to remove moisture. Then the slides were placed in a windmachine that had air from a fan to remove nay loosely held dirtparticles. This step ensured that only strongly bound dirt particleswere retained. These slides were then weighed to determine the excessweight due to deposited dirt. Finally, the transmission spectra of theslides were recorded to determine change light transmission due todeposition of dirt particles. This process was repeated for six cyclesand the results of these are shown in FIGS. 9 and 10.

Various embodiments of the invention have been described above. However,it should be appreciated that alternative embodiments are possible andthat the invention is not limited to the specific embodiments describedabove. Rather, the description of these embodiments should be consideredexemplary of various embodiments that fall within the scope of thepresent invention as defined by the claims.

What is claimed is:
 1. A coating, comprising: a dried gel formed from asol comprising a hydrolyzed alkoxysilane, a hydrolyzed organosilane, anda hydrolyzed organofluorosilane; wherein said dried gel comprisesanti-reflective, abrasion resistant, and water repellant properties. 2.The coating of claim 1, wherein said alkoxysilane comprisestetramethoxysilane, said organosilane comprises methyltrimethoxysilane,and said organofluorosilane comprises trifluoropropyltrimethoxysilane.3. The coating of claim 1, wherein said alkoxysilane comprisestetraethoxysilane, said organosilane comprises methyltrimethoxysilane,and said organofluorosilane comprises trifluoropropyltrimethoxysilane.4. The coating of claim 1, wherein said alkoxysilane comprisestetramethoxysilane, said organosilane comprises methyltrimethoxysilane,and said organofluorosilane comprises(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane.
 5. The coatingof claim 1, wherein said alkoxysilane comprises tetraethoxysilane, saidorganosilane comprises methyltrimethoxysilane, and saidorganofluorosilane comprises(tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane.
 6. The coatingof claim 1, wherein said sol comprises a mixture of a first solcomprising a hydrolyzed alkoxysilane and a second sol comprising amixture of a hydrolyzed organosilane and a hydrolyzedorganofluorosilane.
 7. The coating of claim 1, wherein said solcomprises a relative weight percent ratio of said tetraalkoxysilane to atotal of said organosilane and said organofluorosilane is about 0.2 toabout
 2. 8. The coating of claim 1, wherein said sol comprises arelative weight percent ratio of said organosilane to saidorganofluorosilane of about 0.5 to about 1.5.
 9. The coating of claim 1,wherein said dried gel is formed from a mixture comprising said sol; anorganic solvent selected from the group consisting of alcohols, esters,ethers, aldehydes, and ketones; and water, wherein an amount of theamount of said solvent and said water in said mixture is from about 50%to about 99.5% by weight.
 10. The coating of claim 9, wherein a ratio ofsaid solvent to said water is about 5 to about 20 by weight.
 11. Thecoating of claim 9, wherein said dried gel comprises an abrasionresistance sufficient to pass standard EN 1096.2 abrasion testing whencured at less than 150° C.
 12. The coating of claim 1, wherein said solfurther comprises an acid catalyst in an amount of about 0.001% to about2% by weight and wherein said sol has a pH of about 1 to about
 4. 13.The coating of claim 1, wherein said sol further comprises an basecatalyst in an amount of about 0.001% to about 2% by weight and whereinsaid sol has a pH of about 7 to about
 10. 14. The coating of claim 1,wherein said sol further comprises a low molecular weight polymer in anamount of about 0.1% to about 10% by weight.
 15. The coating of claim 1,wherein said sol further comprises a particle size tuning additive in anamount of about 0.1% to about 2% by weight and that provides atransmission increase of about 0.5% to about 2.0% compared to a samecoating without said particle size tuning additive.
 16. The coating ofclaim 1, wherein said sol further comprises a cross-linking additive.17. The coating of claim 1, wherein said sol further comprises analkoxysilane precursor with functional groups that can react with ahydrophobic reactive agent and wherein said dried gel was exposed to ahydrophobic reactive agent such that said dried gel has an increasedsurface contact angle of up to about 15 degrees compared to a samecoating without said alkoxysilane precursor with functional groups andsaid hydrophobic reactive agent.
 18. The coating of claim 1, furthercomprising a substrate having a surface on which said dried gel isattached and wherein said dried gel has a thickness of about 80 nm toabout 1 micron.
 19. The coating of claim 1 further comprising a solarpanel having a surface on which said dried gel is attached.
 20. Acoating, comprising: a dried gel formed from a sol comprising ahydrolyzed alkoxysilane, a hydrolyzed organosilane, and a hydrolyzedorganofluorosilane; wherein said coating has a contact angle of about 90degrees to about 178 degrees, a pencil hardness of about 2H to about 9H,and an increased transmission of about 1% to about 3.5% compared to asubstrate without said coating.